Add section to wiki explaining how to install the beta version of the app

Wikimedia

While taking screenshots during the whole process, install the Commons Mobile App then switch to the beta, as described in paragraph "Get beta versions of apps" at https://support.google.com/googleplay/answer/7003180?hl=en
Create a Github account
Fill the wiki paragraph https://github.com/commons-app/apps-android-commons/wiki/Volunteers-welcome!#how-to-install-the-beta
Add the screenshots to indicate where to click. Feel free to restart the procedure to take better screenshots if needed.

1337_siddh