This adds documentatuon on transactions and threading concerns.

There are some topics, like application design and conflict resolution
that might want a deeper treatment, but would probably be better
handled through articles or dedicated topics. (I want to avoid
individual topics being too long or weedy to read, where practical.)

Writing this, I stumbled a bit over thread-local transaction managers.
For most applications, they don't add anything over accessing
transaction managers on connections and actually provide an
opportunity to fail. I'm convinced that it should be possible to do
most transaction management through connections and that the API
provided by transaction managers and the transaction package should be
reserved for distributed transactions.

I didn't mention gevent. I thik there should be a section on gevent,
but I think it should be written by someone who's used gevent with
ZODB. :)

Maybe there should also be a section or mention of using asyncio with
ZODB, pr maybe later.

Closes zopefoundation/zodbdocs#13
Closes zopefoundation/zodbdocs#16

Call _p_resolveConflict() even if a conflicting change doesn't change the state

Merge zodbdocs

Don't raise an AttributeError when DemoStorage successfully stores a blob first.

Includes a (previously) failing test case.

Fixes #103.

But we can make it better.

- can ``make html`` from doc directory and get html.

- documentation tests run as part of normal tests.

README.rst needs some love.

Then on to making this work with RTD.

Setup and reference

Reference docs

and fix a ReST issue.

Also the details aren't important here, so moved to a footnote.

Still need to discuss zconfig