[Docs] Add LLVM for Grad Students to Contributing page.

Adrian Sampson's blog post provides a good and relatively up-do-date
introduction to LLVM. I think this post could be helpful for people wanting
to get started with LLVM.

Reviewers: asb, tonic, silvas, probinson, kristof.beyls, rengolin

Reviewed By: rengolin

Differential Revision: https://reviews.llvm.org/D42904

llvm-svn: 326576