Add text about measuring angle

Since QGIS 2.0, it is possible to also measure angles but this is not mentioned in the context help.

resources/context_help/QgsMeasureDialog

@@ -1,6 +1,8 @@
 <h3>Measure Tools</h3>
-There are two measure tools: length and area. With them, you can measure both distances and areas on the map canvas. Currently the tools provide results in metric units only. A single right mouse click stops the measuring, while two right mouse clicks starts measuring anew.
+There are three measure tools: length, area and angle. With them, you can measure both distances, areas and angles on the map canvas. Currently the tools provide results in metric units only. A single right mouse click stops the measuring, while two right mouse clicks starts measuring a new.
 <h4>Measuring Lengths</h4>
 To measure lengths, select the tool and click along the path you want to measure. The length of each segment is displayed as is the total length for the path.
 <h4>Measuring Areas</h4>
 To measure areas, select the tool and click to create the area. The total area is dynamically displayed as you click.
+<h4>Measuring Angles</h4>
+To measure angles, select the tool and click on three points to create an angle between these points. The second point selected is the vertex of the angle. The angle is dynamically displayed once you clicked the second point.