Update TextSecure Staging URL instructions

Small change to clarify where the TextSecure server URL should be changed to the staging value in local builds. 

It looks like the Android variable has been updated from `TEXTSECURE_URL` to `SIGNAL_URL` (ae40715526 (diff-c197962302397baf3a4cc36463dce5eaR179)) and the iOS counterpart is named `textSecureServerURL`.

FREEBIE

CONTRIBUTING.md

@@ -39,9 +39,11 @@ mobile, **you must build a mobile client that targets the staging server**
 ## Linking
 
 
-0. Build Signal for Android or iOS from source, and change its `TEXTSECURE_URL` to point
-   at `textsecure-service-staging.whispersystems.org`. This task is 1% search and
-   replace, 99% setting up your build environment. Instructions are available for both
+0. Build Signal for Android or iOS from source, and point its TextSecure service URL to `textsecure-service-staging.whispersystems.org`:
+  - **on Android:** Replace the `SIGNAL_URL` value in [build.gradle](https://github.com/WhisperSystems/Signal-Android/blob/master/build.gradle)
+  - **on iOS:** Replace the `textSecureServerURL` value in `TSConstants.h`(located in the SignalServiceKit pod)
+  
+    This task is 1% search and replace, 99% setting up your build environment. Instructions are available for both
    the [Android](https://github.com/WhisperSystems/Signal-Android/blob/master/BUILDING.md)
    and [iOS](https://github.com/WhisperSystems/Signal-iOS/blob/master/BUILDING.md) projects.
 1. Upon installing the extension you will be presented with a qr code.