Rewrite README.Git to mention the Gnulib commit number and explain better.

This commit has three purposes.  First, I think that the new version of this
file explains the bootstrapping process a little better.  Second, I want
to mention a Gnulib commit number that I have been using, so that other
people can easily figure out a version of Gnulib that will work.  Third,
I want the PSPP autobuilder to be able to automatically pick out a working
Gnulib commit.