More persistence guides now available

[Dave Hrycyszyn]

Hi everyone,

I've added a few more Scalatra Guides dealing with persistence over the
past few weeks.

There's one on MongoDB:

http://scalatra.org/2.2/guides/persistence/mongodb.html

Riak:

http://scalatra.org/2.2/guides/persistence/riak.html

Also, I've written a guide which goes over the general case. There are
probably a few dozen libraries which *could* be used for persistence
with Scalatra, and the number of specific guides we currently have (4)
doesn't really reflect the richness of options or show how easy it is to
get started with whichever one you choose - so the introduction to
persistence guide gives an overview of the steps for writing your own:

http://scalatra.org/2.2/guides/persistence/introduction.html

If anybody would like to contribute a guide for a specific library which
we don't have docs for, that'd be rather great as well.

Cheers,
Dave