Add tips for working with LLVM.

[ArchRobison]

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.

[tkelman]

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!

[ArchRobison]

Thanks for the education. I'll update the PR.

[tkelman]

Cool. Though your current phrasing

You can override it in your Make.user by adding a line such as:

might not make sense to someone who doesn't know what Make.user means, that file won't exist unless the user creates it. Since this might not be too well-documented elsewhere, or if it is then it's easy to miss, maybe a few more words saying to "Create a file called Make.user and add a line such as ... to it"?

[timholy]

+1!