Version Control for Snippets: Documentation (MVP)

We need to document all the changes we implemented, the new workflows and the existing limitations.

We should add this documentation at the same time we release the feature.

We need to say clear the following things:

• in this stage, only text files are allowed to be created in the repository and that binary files are not supported.
• there is a limit to the number of files in the repository. At the moment, only one file is allowed.
• there is a limit to the size of the file to be pushed. It's the same size limitation we had before when we added the content directly in the UI. This limit is Gitlab::CurrentSettings.snippet_size_limit which, by default, was set to 50 MB.