.github: add CONTRIBUTING.md

Per suggestion from @sfllaw at https://github.com/tailscale/tailscale/pull/15576#issuecomment-2787386082

Updates #engdocs

Change-Id: I67f915db7965ae69dab8925999e7f20208a4269a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/CONTRIBUTING.md

@@ -0,0 +1,17 @@
+PRs welcome! But please file bugs first and explain the problem or
+motivation. For new or changed functionality, strike up a discussion
+and get agreement on the design/solution before spending too much time writing
+code.
+
+Commit messages should [reference
+bugs](https://docs.github.com/en/github/writing-on-github/autolinked-references-and-urls).
+
+We require [Developer Certificate of
+Origin](https://en.wikipedia.org/wiki/Developer_Certificate_of_Origin) (DCO)
+`Signed-off-by` lines in commits. (`git commit -s`)
+
+Please squash your code review edits & force push. Multiple commits in
+a PR are fine, but only if they're each logically separate and all tests pass
+at each stage. No fixup commits.
+
+See [commit-messages.md](docs/commit-messages.md) (or skim `git log`) for our commit message style.