add info about 'type: documentation'

Author: etpinard

CONTRIBUTING.md

@@ -47,6 +47,7 @@ We use the following [labels](https://github.com/plotly/plotly.js/labels) to tra
 | `type: feature` | planned feature additions |
 | `type: performance` | performance related tasks |
 | `type: maintenace` | source code cleanup resulting in no enhancement for users |
+| `type: documentation` | API doc or attribute description improvements |
 | `type: community` | issue left open for community input and pull requests |
 | `type: duplicate` | *self-explanatory* |
 | `type: wontfix` | *self-explanatory* |