README: Add Features section (#73)

The package has an empty readme.
Adding some basic highlights about why this is good
makes sense.

README.md

@@ -2,9 +2,29 @@
 
 `multierr` allows combining one or more Go `error`s together.
 
+## Features
+
+- **Idiomatic**:
+  multierr follows best practices in Go, and keeps your code idiomatic.
+    - It keeps the underlying error type hidden,
+      allowing you to deal in `error` values exclusively.
+    - It provides APIs to safely append into an error from a `defer` statement.
+- **Performant**:
+  multierr is optimized for performance:
+    - It avoids allocations where possible.
+    - It utilizes slice resizing semantics to optimize common cases
+      like appending into the same error object from a loop.
+- **Interoperable**:
+  multierr interoperates with the Go standard library's error APIs seamlessly:
+    - The `errors.Is` and `errors.As` functions *just work*.
+- **Lightweight**:
+  multierr comes with virtually no dependencies.
+
 ## Installation
 
-    go get -u go.uber.org/multierr
+```bash
+go get -u go.uber.org/multierr@latest
+```
 
 ## Status