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))

[hoyosjs]

@oleksandr-didyk I added a commit that makes sure y'all get automatically review requests on changes.

[oleksandr-didyk]

@oleksandr-didyk I added a commit that makes sure y'all get automatically review requests on changes.

Thank you! One more thing though to make the CODEOWNERS entry valid - dotnet/source-build-internal team alias needs write access to the repo (you can see the warning for this if you navigate to the Files changed tab). Without it the alias will not be added to PRs altering the files in the entry