Clarify when repositories.config must be in DVCS

Clarify when repositories.config must be included in source control. Based on experience, https://stackoverflow.com/questions/7286261/nuget-repositories-config, & https://github.com/github/gitignore/pull/1131.

Fixes #175, #13, https://github.com/NuGet/NuGetDocs/pull/92#issuecomment-48626010.