Add 'decades'

decades.md

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+The `decades` table holds information for decade playlists.
+
+### Columns
+| Name                              | Type    | Description                           |
+| --------------------------------- | ------- | ------------------------------------- |
+| decade                            | INTEGER | The decade                            |
+| [propertyid](playlist_properties) | INTEGER | The rowid of this decade's properties |
+
+### Triggers
+| Name                   | When         | Description                                                     |
+| ---------------------- | ------------ | --------------------------------------------------------------- |
+| decades_insert_trigger | After insert | Create a new row in the [[playlist_properties]] table           |
+| decades_delete_trigger | After delete | Delete this decade's entry in the [[playlist_properties]] table |
+
+### The `decades_view`
+The `decades_view` joins the `decades` table with the [[playlist_properties]] table so the application can use one query to access all the relevant state for each decade.
+
+| Column                            | Description                                             |
+| --------------------------------- | ------------------------------------------------------- |
+| decade                            | The decade                                              |
+| [propertyid](playlist_properties) | The rowid of the [[playlist_properties]] for this genre |
+| name                              | The decade formatted as a string                        |
+| [active](playlist_properties)     | The active state of this decade                         |