document microbenchmark requirements, usage

[johnfgibson]

README.md should specify or link to instructions on what the Microbenchmarks require, how to run them, and how to fold their output into julialang.org. In particular specifying and setting up the compilers/interpreters and libraries for all the different languages is pretty non-obvious. I can do this for openSUSE, at least that would be a starting point for other platforms.