Add doc

Author: borkaehw

README.md

@@ -201,6 +201,45 @@ Phases provide 3 major benefits:
 
 See [Customizable Phase](docs/customizable_phase.md) for more info.
 
+## Scala Format
+A phase extension `phase_scalafmt` can format Scala source code via [Scalafmt](https://scalameta.org/scalafmt/).
+
+Add this snippet to `WORKSPACE`
+```
+load("//scala/scalafmt:scalafmt_repositories.bzl", "scalafmt_default_config", "scalafmt_repositories")
+
+scalafmt_default_config()
+
+scalafmt_repositories()
+```
+
+To add this phase to the rules, you have to pass the extension to rule macros. Take `scala_binary` for example,
+```
+load("//scala:advanced_usage/scala.bzl", "make_scala_binary")
+load("//scala/scalafmt:phase_scalafmt_ext.bzl", "ext_scalafmt")
+
+scalafmt_scala_binary = make_scala_binary(ext_scalafmt)
+```
+Then use `scalafmt_scala_binary` as normal.
+
+The extension adds 2 additional attributes to the rule
+ - `format`: enable formatting
+ - `config`: the Scalafmt configuration file
+
+When `format` is set to `true`, you can do
+```
+bazel run <TARGET>.format
+```
+to format the source code, and do
+```
+bazel run <TARGET>.format-test
+```
+to check the format (without modifying source code).
+
+The extension provides default configuration, but there are 2 ways to use custom configuration.
+ - Put `.scalafmt.conf` at root of your workspace
+ - Pass `.scalafmt.conf` in via `config` attribute
+
 ## Building from source
 Test & Build:
 ```