docs: add DroidGround

Author: thelicato

README.md

@@ -227,6 +227,7 @@ A collection of Android security-related resources.
 1. ~~[Opcodes table for quick reference](http://ww38.xchg.info/corkami/opcodes_tables.pdf)~~
 1. ~~[APK-Downloader](http://codekiem.com/2012/02/24/apk-downloader/)~~ - seems dead now
 1. ~~[Dalvik opcodes](http://pallergabor.uw.hu/androidblog/dalvik_opcodes.html)~~
+1. [DroidGround](https://github.com/SECFORCE/droidground) - A flexible playground for Android CTF challenges
 
 ### Vulnerable Applications for practice