Readme, default_, fields.depends, fields.Function

please reject everything i am wrong with..

i added some text about default_values, fields.depends (please correct me if i got the news from 3.2 wrong!!), fields.Function
Also i added a readme to have a list of all available documentation.
maybe this should be put to a different place (fields) at some point...
For the three models a link to the offical doc will be very welcome (using py:class)