Add instructions for usb debugging on android (#343)

Author: trevorah

android/1introduction/tutorial.md

@@ -12,6 +12,9 @@ Studio](https://developer.android.com/studio/index.html) (please
 note: this is a large file, so our advice is to download and install it before
 arriving at the workshop!)
 
+If you want to run your app on your own phone, remember to enable USB debugging in the device system settings, under Settings -> Developer options.
+(please note: On Android 4.2 and newer, Developer options is hidden by default. To make it available, go to Settings -> About phone and tap Build number seven times. Return to the previous screen to find Developer options.)
+
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