Updated README.md to address concern in Issue 105.

https://github.com/trulia/hologram/issues/105

The Quick Start skips an important step. Without this step, new users will see an error which is confusing for someone evaluating the tool.

Warning: Could not generate index.html, there was no content generated for the category basics.
Merge pull request #112 from professor/patch-1

Updated README.md to address concern in Issue 105.