Merge pull request #3069 from johnjohndoe/feature/adb-autocompletion

Add README.md for adb-autocompletion plugin.

plugins/adb/README.md

+# adb autocomplete plugin
+
+* Adds autocomplete options for all adb commands.
+
+
+## Requirements
+
+In order to make this work, you will need to have the Android adb tools set up in your path.