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.
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 ... to it"?