README: document pre-commit (#79)

* README: document pre-commit

This PR documents the pre-commit installation instructions. Do not merge
this PR til v0.8.0

* change version to 0.7.1

README.md

@@ -97,3 +97,16 @@ formatter:
 If the type is not specified, the default formatter will be used. In the tool included in this repo, the default is the [basic formatter](formatters/basic).
 
 For in-depth configuration documentation see the [config docs](docs/config.md).
+
+# pre-commit
+
+Starting in v0.7.1, `yamlfmt` can be used as a hook for the popular [pre-commit](https://pre-commit.com/) tool. To include a `yamlfmt` hook in your `pre-commit` config, add the following to the `repos` block in your `.pre-commit-config.yaml`:
+
+```yaml
+- repo: https://github.com/google/yamlfmt
+  rev: v0.7.1
+  hooks:
+    - id: yamlfmt
+```
+
+When running yamlfmt with the `pre-commit` hook, the only way to configure it is through a `.yamlfmt` configuration file in the root of the repo or a system wide config directory (see [Configuration File](#configuration-file)).