Update benchmark readme with info on how to ensure benchmarks test locally built stdlib

[iThinker]

I recently tried to run modify stdlib and run benchmarks, but I had issues with it, and readme was not helpful enough.
This PR adds a bit of info to readme to help future readers.

Problems addressed:

1. Building standalone benchmarks fails when using standard xcode toolchains. In my case error looked like - error: cannot find '_wordIndex' in scope.
2. I could not make benchmarks use locally built stdlib. Even though this readme says they should use ../lib/swift. So the two env variables helped me solve the problem.

Special thanks to @Catfish-Man for helping with these.

[eeckstein]

The easiest way to run the benchmarks - actually: compare two benchmark builds - is to use the scripts/run_smoke_bench script. This is unfortunately also not documented.
This script e.g. takes care of setting DYLD_LIBRARY_PATH