[DOCUMENTATION] It's not 100% clear how to contribute new benchmarks

There's this CONTRIBUTING file with some details https://github.com/fastruby/fast-ruby/blob/master/CONTRIBUTING.md

But I still have questions that I think it's worth documenting:
- The README says `All results listed in README.md are running with Ruby 2.2.0p0 on OS X 10.10.1. Machine information: MacBook Pro (Retina, 15-inch, Mid 2014), 2.5 GHz Intel Core i7, 16 GB 1600 MHz DDR3. Your results may vary, but you get the idea. : )` so it's not clear how and who should update the readme, maybe that should be removed from there and added for each benchmark so people adding benchmarks can add their specs?
- It's not clear what to do with the Goal described in the Contributing file (for example, it says `At least 12% improvement`, does it mean that a benchmark showing less than that improvement is not accepted / useful?)
- I see code that has methods named `fast` and `slow` (and even `slow2`, `slow3` https://github.com/fastruby/fast-ruby/blob/master/code/proc-and-block/proc-call-vs-yield.rb), I don't think that's the expectation (it makes things harder to read when multiple methods are called `slowX`), I think it could be described better in the CONTRIBUTING file if that's actually the case