Please document the fact that a configured RepositoryUrl interferes with SourceLink

I finally got SourceLink to work after spending a ton of time on it.

The issue was I was trying to enable it on an existing repository that already configured its `` element in each `.csproj` file, which is applied after `true` from the `Directory.Build.props` file in the repository root. After removing the `` element, SourceLink finally started working.

I did a search on the README, but there is not even a mention of `` or the fact that it can interfere with the functionality of SourceLink. Please clarify that in the documentation, as I am sure it will save other people headaches.