README.md: reference website + dev homepages

Change-Id: I1ba9b9f1a3d4ec45055305ddf867c3663c79ae90
Tuleap: #312

README.md

+# libringclient
+
+libringclient is a client library for Ring
+
+For more information about the ring project, see the following:
+- Main website: https://ring.cx/
+- Bug tracker: https://tuleap.ring.cx/projects/ring/
+- Repositories: https://gerrit-ring.savoirfairelinux.com
+
+For build/install instructions, see the INSTALL file.