more parallel query documentation

As previously threatened, I have written some user documentation for
parallel query.  I put it up here:

https://wiki.postgresql.org/wiki/Parallel_Query

This is not totally comprehensive and I can think of a few more
details that could be added, but it's a pretty good high-level
overview of what got into 9.6.  After this has gotten some feedback
and polishing, I would like to add a version of this to the SGML
documentation somewhere.  I am not sure where it would fit, but I
think it's good to document stuff like this.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company