Fixed Jinja2 extensions link

[prokopst]

Apparently Jinja2 docs moved elsewhere, so the old link got broken. This links to hopefully right part of the docs. Feel free to change the link here.

[insspb]

Local tox test with merged #1181:

___________________________________________________________________________________________________________ summary ___________________________________________________________________________________________________________
  py27: commands succeeded
  py35: commands succeeded
  py36: commands succeeded
  py37: commands succeeded
ERROR:   pypy: InterpreterNotFound: pypy
  flake8: commands succeeded

Merge required to start #1179

[ssbarnea]

We are no longer maintaining a list of templates inside README file and instead we use github topics feature. Please read https://github.com/cookiecutter/cookiecutter#a-pantry-full-of-cookiecutters and add the needed topics to your repository and people will be able to find your template. The most important one is cookiecutter-template.

We are also going to remove most of already listed templates from there, in order not avoid being biased.