Add documentation for cache_dir config

Author: ojacques

README.md

@@ -50,6 +50,7 @@ More information about plugins in the [MkDocs documentation][mkdocs-plugins].
 - `branch` - The name of the branch to get contributors from. Example: 'master' (default)
 - `enterprise_hostname` - For GitHub enterprise: the enterprise hostname.
 - `docs_path` - the path to the documentation folder. Defaults to `docs`.
+- `cache_dir` - The path which holds the authors cache file to speed up documentation builds. Defaults to `.cache/plugin/git-committers/`. The cache file is named `page-authors.json.json`.
 
 If the token is not set in `mkdocs.yml` it will be read from the `MKDOCS_GIT_COMMITTERS_APIKEY` environment variable.