Add info about browser incompatibility to etherpad.rst

There is a chrome extension that adds empty lines to an etherpad.
We want to note this extension and discourage its use for
consumers of etherpads.

Change-Id: Ib5a8f4edf17f578c0fbd98cb0b124ff37b557584
Merge "Add info about browser incompatibility to etherpad.rst"