Add instructions about updating docs

[sajith]

docs/README.txt should have some notes on updating documentation and previewing it locally.

[exarkun]

is docs/README.txt a good place for this information? What about someplace _inside_ the documentation? Are folks more likely to find one of these or the other?

A big advantage of putting this _in_ the documentation is that then it can be rst, it shows up on readthedocs (someday), it's in the index, etc.

[sajith]

My guess is that folks are less likely to find this information if we move this inside the manual, at least right now, because contents of the manual are not very discoverable in its current shape. We should re-organize the manual a little bit before moving this content there. I have filed issue #3636 to address this.

I created this ticket merely to make docs/README.txt a little more useful than it is now.

[sajith]

Also created #3639, about adding some content about editing the manual to the manual itself.

[GitHub <noreply@…>]

In b133d0e/trunk:

Merge pull request #997 from sajith/3630.update-docs-readme

Update notes about reading/writing docs

Fixes: ticket:3630