rowanj/gitx#267: Added the missing `bootstrap` step, and dependencies to the read me.

This commit fixes rowanj/gitx#267 by completing the set of dependencies for getting up to speed from scratch.
It can possibly be improved even further by removing Development > Step 4:
A fresh checkout on my machine built just fine after running through steps 1 to 3.

Then again, I’ve been running through the original procedure before, so the update_libgit2 script may have globally installed its artifacts on my system…