update comment on ignoring repositories.config

Wait for https://github.com/NuGet/NuGetDocs/pull/275 to merge before merging.

As mentioned in https://github.com/NuGet/NuGetDocs/pull/262#issuecomment-56222507 "it's actually okay to omit the `repositories.config` from source control, as NuGet will re-generate it when you open the solution again." It appears newer versions of NuGet do not require this file.

/cc @arcresu as you made previous change in https://github.com/github/gitignore/commit/ced2a153472c60d905522041d27874c6a36d60a7 & discussed at https://github.com/github/gitignore/pull/1131#issuecomment-48265224.