Clearing up language about needing holoviews (#152)

examples/Usage.ipynb

@@ -25,7 +25,7 @@
     "\n",
     "5. You are using mac or linux (might work on windows, but currently untested)\n",
     "\n",
-    "6. You are using holoviews for visualization (other packages might work, or not having holoviews might work, but currently untested; if you don't have holoviews, you will at least need to override `nbbuild_ipython_startup` in `nbbuild.py`, setting it to e.g. `\"\"`)."
+    "6. Currently, you need to have holoviews available in your environment. Other packages (bokeh, matplotlib, plotly) can be used for visualization in addition to holoviews, but if you don't have holoviews in the environment, you will at least need to override `nbbuild_ipython_startup` in `nbbuild.py`, setting it to e.g. `\"\"`."
    ]
   },
   {