Added CONTRIBUTING.md file

[jar-stripe]

Why?

Introduce guidelines and tips on how to contribute in order to better enable our development community to make contributions to the SDK.

What?

• add CONTRIBUTING.md and update README.md with a link

[xavdid-stripe]

Nice! some little brevity comments you can take or leave, but I like it.

Also, since nothing in this file is especially ruby-specific, we could have a single copy of it in codegen and copy it over as part of generation. Might need to do some light find-and-replace for the language name to link to issues correctly, but I don't think that's too bad. It'll save us from having to update any typos/changes manually 7 times!

[jar-stripe]

Also, since nothing in this file is especially ruby-specific, we could have a single copy of it in codegen and copy it over as part of generation. Might need to do some light find-and-replace for the language name to link to issues correctly, but I don't think that's too bad. It'll save us from having to update any typos/changes manually 7 times!

I love this idea. I was actually thinking of how to do this, and maintaining a template in codegen that gets copied over as part of generation makes a lot of sense!