DOC: Add link to sphinx-themes.org

Author: mgeier

doc/usage.ipynb

@@ -329,7 +329,8 @@
     "  [example](http://nbsphinx.readthedocs.io/en/jupyter-alabaster-theme/),\n",
     "  [usage](https://github.com/spatialaudio/nbsphinx/compare/jupyter-alabaster-theme^...jupyter-alabaster-theme)\n",
     "\n",
-    "If you know of another Sphinx theme that should be included here, please open an [issue on Github](https://github.com/spatialaudio/nbsphinx/issues)."
+    "If you know of another Sphinx theme that should be included here, please open an [issue on Github](https://github.com/spatialaudio/nbsphinx/issues).\n",
+    "An overview of many more themes can be found at https://sphinx-themes.org/."
    ]
   },
   {
@@ -381,7 +382,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.4rc1"
+   "version": "3.6.4"
   }
  },
  "nbformat": 4,