Document how to do learning with Dynare

See http://www.dynare.org/pipermail/dev/2013-May/003256.html

After discussing with @stepan-a, we decided to keep the claim in the manual introduction that it is possible to do learning with Dynare. There are at least 3 possible types of learning:

• partial information resolution is a form of learning
• it is possible to do least squares learning with extended path
• other types of learning processes can be implemented using a loop over Dynare, as explained by @MichelJuillard in the above post

As a consequence, what is lacking in the documentation is rather examples of how to do learning with Dynare. I therefore rename the ticket, assign it to @stepan-a (who is volunteering), and postponing it.

removed milestone