Add CONTRIBUTING.md

[penberg]

Let's add a CONTRIBUTING.md to make it explicit how to contribute to libsql.

[xonx4l]

You can add things accordingly .

[xonx4l]

As idk what contributing rules you wanna put up.

[redixhumayun]

Hey, is there a Contribution guideline out yet? I'm interested in seeing how I can contribute to this project but couldn't find anything around contributing to it