Merge pull request #265 from buildkite-plugins/update-readme-gitmirror-paths

SUP-1767 Update readme on mount-checkout behavior with git mirrors

Author: pzeballos

README.md

@@ -309,6 +309,8 @@ Specify a file to load a docker image from. If omitted no load will be done.
 
 Whether to automatically mount the current working directory which contains your checked out codebase. Mounts onto `/workdir`, unless `workdir` is set, in which case that will be used.
 
+If there's a git mirror path and `mount-checkout` is enabled, the (mirror path)[https://buildkite.com/docs/pipelines/environment-variables#BUILDKITE_REPO_MIRROR] is mounted into the docker container as an added volume. Otherwise, the git mirror path will have to be explicitly added as an extra volume to mount into the container. 
+
 Default: `true`
 
 ### `mount-buildkite-agent` (optional, boolean)