Add open source contribution guide

[semioticrobotic]

This PR adds a guide for contributing to SAS open source projects on GitHub. It outlines how to get started making contributions, where to find resources, how to comply with our agreements, and what to expect from SAS's open source project maintainers.

[semioticrobotic]

@suehle and @isharacomix: As discussed, here's an initial draft of a SAS contributors' guide. Please take a look and let me know what you might change, add, etc.

Also flagging @cjdinger in case he has feedback to offer. And I'd be interested in @richmain-sas's impressions, too, but for some reason I'm unable to add him as a reviewer.

For context: this guide will be a sub-page of https://sassoftware.github.io/. It will be our "instructions of record" for open source contributors.

What I've committed so far is simply a Markdown file of draft text. When we're confident we've gotten the guide where we'd like it to be, I'll convert this document to HTML and re-circulate for additional review.

[richmain-sas]

I'm not sure why you can't add me as a reviewer. But the doc looks great regardless. Thanks for bringing all of this to fruition! 😎

[semioticrobotic]

Thanks, @richmain-sas! I'll figure it out one of these days. I figured it out. 😏 Regardless, I really appreciate your review!

[semioticrobotic]

@isharacomix, @suehle, @cjdinger: I've converted this to a basic HTML page for easier review, just so you can see what I'm driving at here. It's staged internally at SAS if you'd like to see it in action.

As always, feedback welcome. Next I am working on connecting the primary landing page to the guide.

[semioticrobotic]

Okay! @isharacomix, @suehle, @cjdinger: After working through some internal feedback from @suehle, I think now have what I believe is a workable MVP of the new contributor handbook. This branch reflects all the changes, but I've also staged everything internally at SAS so you can see it in action.

1️⃣ Refreshed home page
2️⃣ The SAS Open Source Contributor Handbook

Please let me know what you think, especially if you have suggestions/requests for changes, ideas for improvements, etc. I'm all ears.

Operating on lazy consensus, if I don't hear objections to the new work in a day or two, I'll merge.

[semioticrobotic]

(Figured I would let @richmain-sas take a look at the page layout, too. 😎)

[cjdinger]

These changes are looking good @semioticrobotic! Just back from being away, and glad to see you didn't wait for my approval to proceed ;)

[semioticrobotic]

Great! Thanks, @cjdinger!

[semioticrobotic]

Also: welcome back!