hg: code-tools/jmh-jdk-microbenchmarks: Add top-level README.md

[jonathan.gibbons at oracle.com]

Changeset: d5faec5671c6
Author:    jjg
Date:      2018-04-18 15:18 -0700
URL:       http://hg.openjdk.java.net/code-tools/jmh-jdk-microbenchmarks/rev/d5faec5671c6

Add top-level README.md
Contributed-by: eric.caspole at oracle.com

+ README.md