README: mention BROWSER_LABEL

I was not sure how to change the browser to use, turns out we simply
have to specify its name with the BROWSER_LABEL:

  export BROWSER_LABEL=phantomjs # Linux/Unix/Mac
  set BROWSER_LABEL=phantomjs # Windows

This adds a mention in the README file.

Change-Id: I54e4d8fc5f8b6b3e6e6a16749b9179a535bff87f
Merge "README: mention BROWSER_LABEL"