README.dissector was split from README.developer in 2013 - update some
documents referring to the split out sections.
Remove trailing whitespace while at this.

Change-Id: I36cfe0ac55e8f653bffbf850e01f582aacf85557
Reviewed-on: https://code.wireshark.org/review/4094
Reviewed-by: Jörg Mayer <jmayer@loplof.de>