Update build instructions

- clarify capability settings

// FREEBIE

BUILDING.md

@@ -70,10 +70,15 @@ Open the `Signal.xcworkspace` in Xcode.
 open Signal.xcworkspace
 ```
 
-In the Signal target on the General tab, change the Team drop down to
-your own. You will need an Apple Developer account for that. On the
-Capabilities tab turn off Push Notifications and Data Protection. Only
-Background Modes should remain on.
+In the TARGETS area of the General tab, change the Team drop down to
+your own. You will need to do that for all the listed targets, for ex. 
+Signal, SignalShareExtension, and SignalMessaging. You will need an Apple
+Developer account for this. On the Capabilities tab, turn off Push 
+Notifications and Data Protection. Background Modes should remain on.
+The App Groups capability will need to remain on in order to access the
+shared data storage. The App ID needs to match the SignalApplicationGroup
+string set in TSConstants.h. The iCloud capability, with iCloud Documents
+selected, will need to be on if you wish to test the Documents API.
 
 Build and Run and you are ready to go!