Add RBS type signature about shale

[kgiszczak]

Would you be willing to take a stab at it? I'd gladly accept PR with this change. If not I'll try to add it myself, although it may take some time.

[sferik]

I forked this repo and created an rbs branch with two commits:

1. Add Steep to typecheck
2. Automatically generate RBS file using TypeProf

When I run bundle exec rake steep, it detects 245 problems in the auto-generated type signatures. I'm not sure I have the expertise in this library necessary to fix them, but I figured I would update this issue with the scaffolding I generated in case that inspires someone else to attempt to resolve these problems.

I believe the majority of these issues can be resolved by adding nil checks or specifying that the value will not be nil in the type signature.

[kgiszczak]

Thank you, that's a good start. I'll try to look into it and maybe add those type signatures when I have some time.