Update pointer to releases in README (#219).

The release informtion has been moved to the releases section on Github, and
old notes are in the `doc` directory under `HISTORY.md`.

This commit updates the URL to release information to point to the releases
section of the Github repository.

README.md

@@ -30,7 +30,7 @@ ikos@lists.nasa.gov
 Release notes
 -------------
 
-See [RELEASE_NOTES.md](RELEASE_NOTES.md)
+See [Releases](https://github.com/NASA-SW-VnV/ikos/releases).
 
 Troubleshooting
 ---------------