Some clarifications for BUILDING.md

Closes #4501
// FREEBIE

BUILDING.md

@@ -5,7 +5,8 @@ Basics
 ------
 
 Signal uses [Gradle](http://gradle.org) to build the project and to maintain
-dependencies.
+dependencies.  However, you needn't install it yourself; the
+"gradle wrapper" `gradlew`, mentioned below, will do that for you.
 
 Building Signal
 ---------------
@@ -19,12 +20,12 @@ The following steps should help you (re)build Signal from the command line.
 2. Make sure you have the [Android SDK](https://developer.android.com/sdk/index.html) installed.
 3. Ensure that the following packages are installed from the Android SDK manager:
     * Android SDK Build Tools
-    * SDK Platform
+    * SDK Platform (API level 22)
     * Android Support Repository
     * Google Repository
-4. Create a local.properties file at the root of your source checkout and add an sdk.dir entry to it.
+4. Create a local.properties file at the root of your source checkout and add an sdk.dir entry to it.  For example:
 
-        sdk.dir=\<path to your sdk installation\>
+        sdk.dir=/Application/android-sdk-macosx
 
 5. Execute Gradle: