Added optional flags field to activity object

- Fixes #545
This is an OK add. The information in these flags is discernable by looking at other data in the payload, so there's no reason not to include the values

docs/topics/Gateway.md

@@ -790,6 +790,7 @@ A user's presence is their current state on a guild. This event is sent when a u
 | assets? | [assets](#DOCS_TOPICS_GATEWAY/activity-object-activity-assets) object | images for the presence and their hover texts |
 | secrets? | [secrets](#DOCS_TOPICS_GATEWAY/activity-object-activity-secrets) object | secrets for Rich Presence joining and spectating |
 | instance? | boolean | whether or not the activity is an instanced game session |
+| flags? | int | [activity flags](#DOCS_TOPICS_GATEWAY/activity-object-activity-flags) `OR`d together, describes what the payload includes |
 
 >info
 >Bots are only able to send `name`, `type`, and optionally `url`.
@@ -836,6 +837,17 @@ A user's presence is their current state on a guild. This event is sent when a u
 | spectate? | string | the secret for spectating a game |
 | match? | string | the secret for a specific instanced match |
 
+###### Activity Flags
+
+| Name         | Value  |
+| ------------ | -------|
+| INSTANCE     | 1 << 0 |
+| JOIN         | 1 << 1 |
+| SPECTATE     | 1 << 2 |
+| JOIN_REQUEST | 1 << 3 |
+| SYNC 				 | 1 << 4 |
+| PLAY 				 | 1 << 5 |
+
 ###### Example Activity
 
 ```json