Mention public 'properties' property

VectorTileFeature has a public 'properties'  property, which was missing from the API documentation.

Author: ahocevar

README.md

@@ -93,6 +93,7 @@ An object that contains the data for a single feature.
 
 - **type** (`Number`) — type of the feature (also see `VectorTileFeature.types`)
 - **extent** (`Number`) — feature extent size
+- **properties** (`Object`) — object literal with feature properties
 
 #### Methods