add play store link

docs/software/android-too-old.md

@@ -6,6 +6,8 @@ If you've landed here that means your android application is too old for the run
 
 Probably, what this means is that you installed the **alpha test** version of the firmware from github.  We really love people helping with development by running the alpha test binaries.  But if you aren't ready to sign up for that right now, please go back to [github](https://github.com/meshtastic/Meshtastic-device/releases) and install the latest **not alpha** 1.1.x firmware.
 
-If you **do** intend to run the alpha test please opt-in to receive the alpha test version of the android application.
+If you **do** intend to run the alpha test please [opt-in](https://play.google.com/apps/testing/com.geeksville.mesh) to receive the alpha test version of the android application.
+
+If you are willing to be an alpha tester, please keep an eye on our forum where we post frequent release notes.  We also will actively help you with any bugs you might encounter (along our shared journey of new feature goodness).
 
 If you have problems/questions please post in our [forum](https://meshtastic.discourse.group) and some nice person will probably help.