Add the list of existing benchmarks to the top of README

[dumblob]

Currently it's not easy at all to find the benchmarks. Could you please restructure README such, that at the very beginning (at least before the table with results is presented) there is a list of all existing benchmarks, which are available for all the benchmarked languages? Also a direct URL pointing to the the tests/ directory is a must have.

Without this the benchmarks do not have unfortunately any value for the reader 😢.