Add reference to 18.04 test framework documentation
Test Framework Documentation {#test_framework_doc}
============================
-PyDoc generated documentation for the "make test" framework is available for the following releases
+PyDoc generated documentation for the "make test" framework is available for
+the following releases:
+- [Test framework documentation for VPP 18.04](https://docs.fd.io/vpp/18.04/vpp_make_test/html)
- [Test framework documentation for VPP 18.01](https://docs.fd.io/vpp/18.01/vpp_make_test/html)
- [Test framework documentation for VPP 17.10](https://docs.fd.io/vpp/17.10/vpp_make_test/html)
- [Test framework documentation for VPP 17.04](https://docs.fd.io/vpp/17.04/vpp_make_test/html)
