README: add Contributing bullet

People will often look for contributing instructions from
https://github.com/martinvonz/jj, this makes it easier
to find.

It will also lead more people to read the contributing
docs on the website as opposed to GitHub. This is becoming
desireable, see the discussion at
#4822 (comment)

README.md

@@ -11,7 +11,8 @@
 **[Homepage]   •  **
 **[Installation]   •  **
 **[Getting Started]   •  **
-**[Development Roadmap]**
+**[Development Roadmap]   •  **
+**[Contributing](#contributing)**
 
 [Homepage]: https://martinvonz.github.io/jj
 [Installation]: https://martinvonz.github.io/jj/latest/install-and-setup