Add draft contributor guidelines

[atomb]

This PR includes a draft CONTRIBUTING.md with guidelines for developing with SAW. I imagine it's missing or misstating things, as-is, but opening it up for review is probably the quickest way to resolve that.

[kquick]

As a general comment, adding the CODE_OF_CONDUCT add specific obligations that we are required to fulfill. I'm personally not opposed to the principles here in any way, but by explicitly stating them we are essentially creating a contract; we might want to have our legal/contracts folks verify this before posting it.

[lisanna-dettwyler]

As a general comment, adding the CODE_OF_CONDUCT add specific obligations that we are required to fulfill. I'm personally not opposed to the principles here in any way, but by explicitly stating them we are essentially creating a contract; we might want to have our legal/contracts folks verify this before posting it.

Maybe it's also something we have a larger conversation about? Also not opposed to having them but it might be one of those things that some people might be more opinionated about than others, and having our own galois-centric one could be nice.

[lisanna-dettwyler]

At some point soon I'd like to add a section to CONTRIBUTING.md showing how to get different kinds of changes integrated into master with feature flags, so people don't need to have long-lived nightmare branches/forks.

[lisanna-dettwyler]

Dead link