DOC: Add instructions to install pandoc

Closes #72.

doc/usage.ipynb

@@ -29,6 +29,19 @@
     "Recent versions of Python already come with `pip` pre-installed.\n",
     "If you don't have it, you can [install it manually](https://pip.pypa.io/en/latest/installing/).\n",
     "\n",
+    "### pandoc\n",
+    "\n",
+    "The stand-alone program [pandoc](http://pandoc.org/) is used to convert Markdown content to something Sphinx can understand. You have to install this program separately, ideally with your package manager. If you are using `conda`, you can select the [conda-forge](https://conda-forge.github.io/) channel to install the [pandoc](https://anaconda.org/conda-forge/pandoc) package.\n",
+    "If that doesn't work out for you, have a look at `pandoc`'s [installation instructions](http://pandoc.org/installing.html).\n",
+    "\n",
+    "<div class=\"alert alert-info\">\n",
+    "\n",
+    "**Note:**\n",
+    "\n",
+    "The use of `pandoc` in `nbsphinx` is temporary, but will likely stay that way for a long time, see [issue #36](https://github.com/spatialaudio/nbsphinx/issues/36).\n",
+    "\n",
+    "</div>\n",
+    "\n",
     "### Syntax Highlighting\n",
     "\n",
     "To get proper syntax highlighting in code cells, you'll need an appropriate *Pygments lexer*.\n",