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...
so that interpretation is correct?
  - in on_change_\<field\>: fields.depends() states which fields have to be provided by the client to call the function
  - in on_change_with_\<field\> it states which fields trigger a call

what i see in the code it just adds the fields names to the functions depends attribute, but this doesnt yet clarify how the trigger-fields are set in on_change_with.

thanks for being so patient :)