README doesn't explain that this isn't maintained any more

[gdt]

I recently found that chatsecure was not receiving messages because of #1250.

This issue is not a complaint about the state of maintenance, but more narrowly focused. If, as seems to be the case, ChatSecure is no longer usable because of push problems, the README should say that it has reach end-of-life and should not longer be used. Or perhaps an explanation of how one works around broken push. As it is, the README makes it look usable.