Fix the link error in the document README.rst

1. What is the problem?
The Tricircle Admin API documentation has been implemented, so the
relevant link to it in the document README.rst should be updated as
well.

2. What is the solution to the problem?
Remove the original null link at the bottom of the document README.rst and
change the part "Documentation" to "Tricircle Admin API documentation", then
provide the relative link behind it.

3. What the features need to be implemented to the Tricircle to realize the solution?
None.

Change-Id: I8dae02ddef4a3019abe91ec1ca283280b64c7c61

README.rst

@@ -31,7 +31,7 @@ specific bottom OpenStack instances in one or multiple AZs automatically.
 * Design documentation: `Tricircle Design Blueprint <https://docs.google.com/document/d/18kZZ1snMOCD9IQvUKI5NVDzSASpw-QKj7l2zNqMEd3g/>`_
 * Wiki: https://wiki.openstack.org/wiki/tricircle
 * Installation with DevStack: https://github.com/openstack/tricircle/blob/master/doc/source/installation.rst
-* Documentation: TBD
+* Tricircle Admin API documentation: https://github.com/openstack/tricircle/blob/master/doc/source/api_v1.rst
 * Source: https://github.com/openstack/tricircle
 * Bugs: http://bugs.launchpad.net/tricircle
 * Blueprints: https://launchpad.net/tricircle