Add/restore CONTRIBUTING.md

It just links to our actual docs and repeats the passage from README.
This should cause GitHub to have a "Contributing" tab on
https://github.com/martinvonz/jj, which will make these instrictions a
lot easier to find.

This will also hopefully mean that more people read the instructions on
our website rather than GitHub, which will be a somewhat nicer
experience after #4822.

CONTRIBUTING.md

@@ -0,0 +1,16 @@
+Thanks for your interest for contributing to `jj`! See <https://martinvonz.github.io/jj/prerelease/contributing/>
+for detailed instructions[^md].
+
+[^md]: Those docs are generated from
+[`docs/contributing.md`](docs/contributing.md).
+
+The broad TL;DR is:
+
+<!-- Keep this section in sync with https://github.com/martinvonz/jj?tab=readme-ov-file#contributing -->
+
+- Bug reports are very welcome!
+- Every commit that lands in the `main` branch is code reviewed.
+- Please behave yourself, and obey the Community Guidelines.
+- There **is** a mandatory CLA you must agree to. Importantly, it **does not**
+  transfer copyright ownership to Google or anyone else; it simply gives us the
+  right to safely redistribute and use your changes.

README.md

@@ -322,6 +322,8 @@ We do have [a few policies and
 suggestions](https://martinvonz.github.io/jj/prerelease/contributing/)
 for contributors. The broad TL;DR:
 
+<!-- Keep this section in sync with CONTRIBUTING.md -->
+
 - Bug reports are very welcome!
 - Every commit that lands in the `main` branch is code reviewed.
 - Please behave yourself, and obey the Community Guidelines.