add android app alpha tester channel

docs/README.md

@@ -20,6 +20,11 @@ Soon our first alpha release of will be released here:
 
 [![Download at https://play.google.com/store/apps/details?id=com.geeksville.mesh](https://play.google.com/intl/en_us/badges/static/images/badges/en_badge_web_generic.png)](https://play.google.com/store/apps/details?id=com.geeksville.mesh&referrer=utm_source%3Dhomepage%26anid%3Dadmob)
 
+If you would like to join our bleeding-edge alpha test group for this app, we'd love to have you.  Three steps:
+1. Join [this Google group](https://groups.google.com/forum/#!forum/meshtastic-alpha-testers) with the account you use in Google Play.
+2. Go to this [URL](https://play.google.com/apps/testing/com.geeksville.mesh) to opt-in to the alpha test.
+3. If you encounter any problems or have questions, post in our gitter chat and we'll help.
+
 ## Supported hardware
 We currently support two brands of radios.  The [TTGO T-Beam](https://www.aliexpress.com/item/4000119152086.html) and the [Heltec LoRa 32](https://heltec.org/project/wifi-lora-32/).  Most users should buy the T-Beam and a 18650 battery (total cost less than $35).  Make
 sure to buy the frequency range which is legal for your country.  For the USA, you should buy the 915MHz version.  Getting a version that include a screen