Document CODE_COMMIT_ENDPOINT usage

Hi,

We have a specific case where access do CodeCommit should be made through an Endpoint URL with temporary credentials.

After a lot of unsuccessful tries, we found and undocumented feature where the Endpoint URLl could be configured using an environment variable named `CODE_COMMIT_ENDPOINT`.

https://github.com/aws/git-remote-codecommit/blob/c696b4977761ea5b0c0e385da69a0bd09034b566/git_remote_codecommit/__init__.py#L204

I tried to find a documentation for this feature, but have no success. So I create this issue to suggest creating this documentation.