docs(README): update repository README screenshots and help text (#422)

In #421, the screenshots in the `tokio-console` README were updated to
match the current code. The contents of the README also go into the
docs.rs page.

This change updates the two screenshots in the README in the root of the
repository. They now match the UI shown by the current code at the
latest release of `tokio-console`, 0.1.8. The screenshots are a little
bit wider than the ones in the `tokio-console` README as the example
`app` from the `console-subscriber` crate was used as the instrumented
application and the source file paths are longer.

The screenhots are now using relative links directly to the files in the
repository.

Additionally, the help text in the main README has been updated to
reflect what `tokio-console` now produces. This includes additional
descriptive text and also mentions how to connect to a Unix domain
socket.

Author: hds

README.md

@@ -53,11 +53,11 @@ toolkit consists of multiple components:
 
 wow! whoa! it's like `top(1)` for tasks!
 
-![task list view](https://user-images.githubusercontent.com/2796466/129774465-7bd2ad2f-f1a3-4830-a8fa-f72667028fa1.png)
+![task list view](assets/readme/top-for-tasks.png)
 
 viewing details for a single task:
 
-![task details view](https://user-images.githubusercontent.com/2796466/129774524-288c967b-6066-4f98-973d-099b3e6a2c55.png)
+![task details view](assets/readme/task-details.png)
 
 ## on the shoulders of giants
 
@@ -163,58 +163,75 @@ as an argument to the console (either as an `<IP>:<PORT>` or
 cargo run -- http://my.great.console.app.local:5555
 ```
 
-the console command-line tool supports a number of additional flags to configure
-its behavior. the `-h` or `--help` flag will print a list of supported
-command-line flags and arguments:
+The console command-line tool supports a number of additional flags to configure
+its behavior. The `help` command will print a list of supported command-line
+flags and arguments:
 
 ```text
 USAGE:
-    tokio-console [FLAGS] [OPTIONS] [TARGET_ADDR]
+    tokio-console [OPTIONS] [TARGET_ADDR] [SUBCOMMAND]
 
 ARGS:
     <TARGET_ADDR>
             The address of a console-enabled process to connect to.
 
-            This may be an IP address and port, or a DNS name. [default: http://127.0.0.1:6669]
+            This may be an IP address and port, or a DNS name.
 
-FLAGS:
-        --ascii-only
-            Explicitly use only ASCII characters
-
-    -h, --help
-            Print help information
-
-        --no-colors
-            Disable ANSI colors entirely
+            On Unix platforms, this may also be a URI with the `file` scheme that specifies the path
+            to a Unix domain socket, as in `file://localhost/path/to/socket`.
 
-        --no-duration-colors
-            Disable color-coding for duration units
-
-        --no-terminated-colors
-            Disable color-coding for terminated tasks
-
-    -V, --version
-            Print version information
+            [default: http://127.0.0.1:6669]
 
 OPTIONS:
+        --ascii-only <ASCII_ONLY>
+            Explicitly use only ASCII characters
+
         --colorterm <truecolor>
             Overrides the value of the `COLORTERM` environment variable.
 
             If this is set to `24bit` or `truecolor`, 24-bit RGB color support will be enabled.
-            [env: COLORTERM=truecolor] [possible values: 24bit, truecolor]
+
+            [env: COLORTERM=truecolor]
+            [possible values: 24bit, truecolor]
+
+    -h, --help
+            Print help information
 
         --lang <LANG>
-            Overrides the terminal's default language [env: LANG=en_US.UTF-8] [default: en_us.UTF-8]
+            Overrides the terminal's default language
+
+            [env: LANG=]
 
         --log <ENV_FILTER>
             Log level filter for the console's internal diagnostics.
 
-            The console will log to stderr if a log level filter is provided. Since the console
-            application runs interactively, stderr should generally be redirected to a file to avoid
-            interfering with the console's text output. [env: RUST_LOG=] [default: off]
+            Logs are written to a new file at the path given by the `--log-dir` argument (or its
+            default value), or to the system journal if `systemd-journald` support is enabled.
+
+            If this is set to 'off' or is not set, no logs will be written.
+
+            [default: off]
+
+            [env: RUST_LOG=]
+
+        --log-dir <LOG_DIRECTORY>
+            Path to a directory to write the console's internal logs to.
+
+            [default: /tmp/tokio-console/logs]
+
+        --no-colors
+            Disable ANSI colors entirely
+
+        --no-duration-colors <COLOR_DURATIONS>
+            Disable color-coding for duration units
+
+        --no-terminated-colors <COLOR_TERMINATED>
+            Disable color-coding for terminated tasks
 
         --palette <PALETTE>
-            Explicitly set which color palette to use [possible values: 8, 16, 256, all, off]
+            Explicitly set which color palette to use
+
+            [possible values: 8, 16, 256, all, off]
 
         --retain-for <RETAIN_FOR>
             How long to continue displaying completed tasks and dropped resources after they have
@@ -243,7 +260,21 @@ OPTIONS:
 
             * `months`, `month`, `M` -- defined as 30.44 days
 
-            * `years`, `year`, `y` -- defined as 365.25 days [default: 6s]
+            * `years`, `year`, `y` -- defined as 365.25 days
+
+            [default: 6s]
+
+    -V, --version
+            Print version information
+
+SUBCOMMANDS:
+    gen-completion
+            Generate shell completions
+    gen-config
+            Generate a `console.toml` config file with the default configuration values, overridden
+            by any provided command-line arguments
+    help
+            Print this message or the help of the given subcommand(s)
 ```
 
 ## for development