VCS README example - point out .rst format.

[sypets]

https://docs.typo3.org/m/typo3/docs-how-to-document/main/en-us/UserRoundTrip.html#vcs-repository

Example README for VCS repository: please point out what format this is (.rst or .md). It is possible to find this out but I think it should be stated more explicitly. People are not always aware of the distinction and even not aware that you can use .rst (and not .md) as README on GitHub / Gitlab etc.

Also, optionally also offer a Markdown example. Some (many) extension authors use Markdown as README and will/may not change this.