Add explicit discussion of dependencies to README.rst

Noted that building the documentation has more dependencies
than merely running Nova.
Noted that the standard development environment is sufficient.
Noted that the standard runtime environment plus oslo.sphinx
is sufficient.

Change-Id: I37b84a1bdfbc906b005c0a53c2c570c0695937b3
Closes-Bug: #1266623
Merge "Add explicit discussion of dependencies to README.rst"