Clarify "autogenerated" option for columns

[jwoertink]

If your column gets a default value from the database, you need to use the autogenerated: true column option or specify a default value in Crystal...

column admin : Bool, autogenerated: true
# or
column admin : Bool = false

If you don't specify one of these, then when trying to save through an operation, you'll get an error about a missing column unless you pass in a value.

We should add more documentation about this.