New users not told to disable the built-in camera to prevent a crash when used from the lockscreen

[hamishmb]

As per the discussion at #281, Simple Camera now works fine from the lockscreen, but only if the default camera is disabled.

This isn't noted in README.md, and also not displayed on first launch of the app - this is important because many users will probably get the app from F-Droid, and those users probably won't see the readme anyway.

I'll submit a MR now to update README.md.