[MERGE] Add a brief Portability section to HACKING, build HACKING.htm from 'make docs'.

[Alexander Belchenko]

Alexander Belchenko has voted +1 (conditional).
Status is now: Approved
Comment:
If this HACKING.htm is intended to be published on doc.bazaar-vcs.org, 
then you need also update sources for target '$(PRETTYDIR)/%.htm'

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20070412054524.GF6345%40steerpike.home.puzzling.org%3E