Add documentation building to README
Add documentation building to README

This change updates the README with the steps to build and open the
upstream documentation.

Change-Id: Ib12b0d30cc6f984e1f0f01fd76466024bc21b69c

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed