Update cookiecutter link in README

The existing link, https://cookiecutter.readthedocs.io/, takes you
to an old version of the cookiecutter documentation, where you are
warned, "You are not reading the most recent version of this
documentation."

This appears to be a bug in readthedocs.io or cookiecutter's site
itself; even their own latest documentation home page,

https://cookiecutter.readthedocs.io/en/latest/README.html,

links to https://cookiecutter.readthedocs.io/, which in turn takes
you back to that same old page for version 1.7.2.

Author: unusualinsights

README.md

@@ -51,7 +51,7 @@ You must also read "Your first commit" (below) before continuing further:
 
 (Skip this section if you chose to click GitHub's "Use this template" button.)
 
-Get [cookiecutter](https://cookiecutter.readthedocs.io/).
+Get [cookiecutter](https://cookiecutter.readthedocs.io/en/latest/).
 
 Pull down our cookiecutter-ized branch using cookiecutter directly, like so: