Fix broken doc links in DEVELOPER.md
Fix broken doc links in DEVELOPER.md

The current links 404 when reading the developer instructions from GitHub.

These new one use relative links instead of absolute links; they now resolves to the actual hosted page at e.g. https://github.com/candid82/joker/blob/master/LIBRARIES.md and should also work within branches if browsing non-`master` branches.
candid82/joker,candid82/joker,candid82/joker