add review comment to sb files

[oleksandr-didyk]

Contributes to dotnet/source-build#3435

Adds comments to source-build files asking for the inclusion of the source-build team in PRs that alter SourceBuild* files. Non-reviewed changes could potentially cause issues down the line, be it in the downstream repos or the product build (as has happened in the past, see dotnet/source-build#3435 (comment))

[oleksandr-didyk]

@tmat soft ping - would be great if you could take a look at this small PR. Thank you!

[tmat]

Wanna add .github/CODEOWNERS file? E.g. https://github.com/dotnet/aspnetcore/blob/main/.github/CODEOWNERS

[oleksandr-didyk]

Wanna add .github/CODEOWNERS file? E.g. dotnet/aspnetcore@main/.github/CODEOWNERS

Originally I wanted to add both the comments and the CODEOWNERS entry with dotnet/source-build-internal, but for a team alias entry to be valid the team needs to have write access to the repository.

If you are OK with grating write access to dotnet/source-build-internal then I will add the file. Otherwise the comments are currently the easiest / cheapest way to guard against breaking edits