Fix some text in Air.js/README.md

Fix some text in Air.js/README.md
https://bugs.webkit.org/show_bug.cgi?id=158650

Reviewed by Benjamin Poulain.

I read the text again and found bugs:

- We never actually say how to run the benchmark. This change adds a blurb about how to run
  it.

- We both say that allocateStack is responsible for the bulk of the running time and that
  we haven't measured where the bulk of the time is spent. This changes the text to say that
  it was a goal to make allocateStack be the hottest part of the benchmark, but that we did
  not measure this.

* Air.js/README.md:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@201959 268f45cc-cd09-0410-ab3c-d52691b4dbfc