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

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> 
> ^- I would have rather broken this up a bit (putting the osutils warning
> into a subsection).

When this section contains more than three sentences, then we can look at having
subsections...

[...]
> But I think this is still a step forward.

Right.  There's a lot more to do, but this is a start.  With some luck, even
this small note might save Alexander some grief :)

-Andrew.