Simplified the README file.

Detailed documentation was moved to a separate repository
in order to keep the main repository clean from lots of
documentation edits.

See: http://hg.nginx.org/unit-docs

README

@@ -0,0 +1,19 @@
+The documentation and binary packages are available online:
+
+  http://unit.nginx.org
+
+The source code is provided under the terms of Apache License 2.0:
+
+  http://hg.nginx.org/unit
+
+Please ask questions, report issues, and send patches to the mailing list:
+
+  unit@nginx.org (http://mailman.nginx.org/mailman/listinfo/unit)
+
+or via Github:
+
+  https://github.com/nginx/unit
+
+--
+NGINX, Inc.
+http://nginx.com