Add ANDROID_SDK_ROOT to Android's caveats.

Formula/android-sdk.rb

@@ -25,9 +25,13 @@ class AndroidSdk <Formula
     end
   end
 
-  def caveats; "\
+  def caveats; <<-EOS
 We agreed to the Android SDK License Agreement for you by downloading the SDK.
 If this is unacceptable you should uninstall.
-You can read the license at: http://developer.android.com/sdk/terms.html"
+You can read the license at: http://developer.android.com/sdk/terms.html
+
+Please add this line to your .bash_profile:
+  export ANDROID_SDK_ROOT=#{prefix}
+EOS
   end
 end