doc/ext: add link to Git Version Control Cookbook

This is newly published. I haven't read it, but our general
strategy seems to be to link to as many books as possible
from the external links page.
Merge pull request #441 from git/cookbook-link

doc/ext: add link to Git Version Control Cookbook
Rails 4.1.5
Fix missing end keyword
Remove vim stuff
Rework on the asciidoctor
Rspec is behaving werid
Import more txt files
Fix links
Update regex
Include git docs
Redirect to howto file