Give examples of calling stored procedures

[d6y]

Re: https://github.com/underscoreio/essential-slick-code/issues/16

It's a big topic, especially regarding error handling (which I'm guessing is going to be dependent on the database in use).

It probably mainly belongs in the Plain SQL chapter, but should possibly be called out earlier (perhaps data modelling?)

[d6y]

Example: https://github.com/d6y/slick-postgresql-stored-proc