Tweak debian/README.source further.
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 buildd machines.

Author: 3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@pocock.pro

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