Update and simplify README with copy consistent with the wiki.

Also add a GitHub specific version with a screenshot, links, and a note
about not using tickets and pull requests.

https://bugzilla.gnome.org/show_bug.cgi?id=775215

README.md

@@ -0,0 +1,43 @@
+
+Geary: Send and receive email
+=============================
+
+Geary is an email application built around conversations, for the
+GNOME 3 desktop. It allows you to read, find and send email with a
+straightforward, modern interface.
+
+Visit https://wiki.gnome.org/Apps/Geary for more information.
+
+**GitHub users please note**: Bug reports, code contributions and
+translations are managed using GNOME's infrastructure, so we cannot
+accept tickets or pull requests on GitHub. Please see the links below
+for more information.
+
+![Geary displaying a conversation](https://wiki.gnome.org/Apps/Geary?action=AttachFile&do=get&target=geary-conversation-0.12.png)
+
+Installation & Licensing
+------------------------
+
+Please consult the [INSTALL](./INSTALL) and [COPYING](./COPYING) files
+for more information.
+
+Contributing to Geary
+---------------------
+
+Want to help improve Geary? Here are some ways to contribute:
+
+ * Bug reporting: https://wiki.gnome.org/Apps/Geary/ReportingABug
+ * Development:   https://wiki.gnome.org/Apps/Geary/Development
+ * Translating:   https://wiki.gnome.org/Apps/Geary/Translating
+ * Join the mailing list or IRC channel and join in the discussion
+
+Getting in Touch
+----------------
+
+ * Geary wiki:   https://wiki.gnome.org/Apps/Geary
+ * Mailing list: http://mail.gnome.org/mailman/listinfo/geary-list
+ * IRC Channel:  [irc://irc.gimp.org/%23geary](#geary on irc.gimp.org)
+
+---
+Copyright 2016 Software Freedom Conservancy Inc.  
+Copyright 2017 Michael Gratton <mike@vee.net>