Added description of YAML usage to docs.
Added description of YAML usage to docs.

Added section to the docs that points out that YAML, JSON, etc. can be
used to define schemas.

MacHu-GWU/cerberus-sanhe,funkyfuture/cerberus,dkellner/cerberus,calve/cerberus,nicolaiarocci/cerberus,nicolaiarocci/cerberus,dkellner/cerberus,pyeve/cerberus,pyeve/cerberus,funkyfuture/cerberus,MacHu-GWU/cerberus-sanhe,calve/cerberus