Add short README.md for the repository

Author: juliandescottes

README.md

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+## ADB Extension
+
+This is a Firefox extension that supports remote debugging in Firefox DevTools.
+It provides ADB binaries used by DevTools to connect to Firefox/GeckoView products on Android devices via USB.
+
+### Releases
+
+For documentation about releases check out [RELEASE.md](./RELEASE.md)
+
+### Discussion
+
+For questions and issues specific to this extension, you can use the [GitHub issue tracker](https://github.com/mozilla/devtools-adb-extension/issues).
+
+For more general questions about remote debugging in DevTools or DevTools in general, you can:
+- come chat with us on [Slack](https://devtools-html-slack.herokuapp.com/) or IRC (#devtools at irc.mozilla.org)
+- file bugs on [Bugzilla](https://bugzilla.mozilla.org/enter_bug.cgi?product=DevTools)