Add tips for working with LLVM.

This PR adds Julia-specific advice about LLVM to  `devdocs`, culled from my notes from working on Julia.  Hopefully others can extend it in the future.
Generally it's a better recommendation to create a `Make.user` file that contains `override LLVM_VER = svn` or similar, rather than editing `deps/Versions.make`. That way your local git copy won't be dirty.

Very useful notes, though, thanks!