Add topic for "critical distance" unit

Author: mgeier

index.ipynb

@@ -42,7 +42,7 @@
     "\n",
     "1. [Stereophonic Recording Techniques](http://nbviewer.ipython.org/github/spatialaudio/communication-acoustics-exercises/blob/master/recording.ipynb)\n",
     "\n",
-    "1. ...\n",
+    "1. Room Acoustics, Critical Distance (no notebook available)\n",
     "\n",
     "1. Revision (no notebook available)"
    ]
@@ -92,7 +92,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.4.4"
+   "version": "3.5.1+"
   }
  },
  "nbformat": 4,