Add CONTRIBUTING guide

[elskwid]

Overview

Adds initial contributing guide.

Details

Adds a starting point for our contributing guide focused on the basic process of:

1. Gaining a little background information for how community contributions are handled
2. Thinking about how this project might look a little different than the raw specifications
   1. I lifted a section from the Python guide that talks about "capabilities not compliance" as result of this discussion
3. The basic mechanics of contributing

Things we could add

There might be some other things we could add in here like:

• Any design rationale for standout difference between this and other libraries or the spec
• Links or information on style: commits, language, etc
• RuboCop usage (or expected usage)
• Bug reports (I notice that the JS folks have a section on this and it seems like it could be helpful in the future)

[fbogsany]

Thanks @elskwid! I like this as is. I also like the "things we could add", but I'd be happy to merge this as is and open issues to add those things. I've been thinking recently about the first one, since we have some distinct Ruby-isms in our codebase that differ significantly from other implementations.

[elskwid]

@fbogsany Thanks for the review! I'm perfectly happy with a merge now. I can create issues for the "things we could add" and we can set aside time later on.

[elskwid]

🙇

I added #105 (design rationale), #106 (style), #107 (bug reports).