Add note about maintenance mode to readme

Both @dschuermann and myself no longer spend a lot of time on this
project.

Here's a brief summary of a blog post we never wrote:

* All of OpenKeychain's UI is an anti-feature.
* If the user is doing anything in OpenKeychain's UI, OpenPGP is still doing it wrong.
* OpenKeychain shouldn't be an app, it should be a library.
* Changing OpenKeychain into a library is more work than we are motivated to do in our free time.

README.md

@@ -1,3 +1,7 @@
+**WARNING: This software is no longer actively developed.**
+We will still apply security fixes where reported, and do basic maintenance work, but no new features or will be worked on.
+We will try to consider and merge contributions where possible.
+
 # OpenKeychain (for Android)
 
 OpenKeychain is an OpenPGP implementation for Android.