Add example of adding/removing column to existing table

[jsyrjala]

Documentation should have examples of how to alter table add/remove column. Currently that is not too clear.

[jjttjj]

Took me a bit of playing around at first but here is a working example:

(defmigration add-data-to-videos
  (up []
      (alter :add
             (table :videos
                    (int :duration)
                    (double :frame_rate))))
  (down []
        (alter :drop
               (table :videos
                      (column :duration)
                      (column :frame_rate)))))

[burningion]

Agreed. Thanks for your input, it helped get me up and running.

[graue]

Had to google for this. Agree that an example like this in the docs would be helpful.