Added instructions for LLVM and LibFFI by default to README
Add example of custom.mk for building with both optional dependencies
Add note about custom.mk example
Merge remote-tracking branch 'upstream/master' into readme-llvm-ffi to fix reg016.
Merge pull request #603 from david-christiansen/readme-llvm-ffi

Update README with info about optional dependencies
Merge branch 'master' of https://github.com/idris-lang/Idris-dev