Add automation to prevent PRs from changing specification files that need to be immutable.

[darrelmiller]

It is very hard for people to know which specification file to update when making changes. If people do mistakenly try and change the wrong file we should prevent that and provide guidance as to which is the correct file to edit.

[kevinswiber]

I can help with this. Which file paths would we like to restrict? Would we want an override mechanism?

[Bellangelo]

This issue needs some extra clarifications. For example, what files should we forbid? Should we forbid the files that we auto-generate, like the produced JSON spec files? Also, if we need to forbid extra files, do we need to forbid them based on the branch that you are about to merge? For example, you can update 3.1.0 spec only when you are about to merge your PR in the 3.1.0-dev branch.