docs: add contributing guide with branch-based workflow · timvw/wt@d6e120a

Failed to save: <h2>Our services aren't available right now</h2><p>We're working to restore all services as soon as possible. Please check back soon.</p>0PQsraQAAAABndXMuec70Q5Bu6u07vOHXUEFPRURHRTA1MTUARWRnZQ==

Failed to restore: Cache service responded with 400

Failed to restore: "/usr/bin/tar" failed with error: The process '/usr/bin/tar' failed with exit code 2