Tweak debian/README.source further.

Add comments about Build-Depends-Indep.

If somebody tries to specify Build-Depends-Indep then they will find the
package fails to build on the Debian build machines.

Author: daniel@pocock.pro

git-svn-id: http://libphonenumber.googlecode.com/svn/trunk@691 ee073f10-1060-11df-b6a4-87a95322a99c