Add documentation on how to upgrade libraries to latest template version using cruft

[cmutel]

Using e.g. https://pypi.org/project/cookiecutter-project-upgrader/

[tngTUDOR]

Another option would be to use cruft as the rendering engine, and profit from it's update feature.

[pros]

• fully compatible with cookiecutter templates
• can be used with "already generated by cookiecutter" projects
• uses git commit id as a means to identify which version of the cookiecutter template was originally used

[cons]

• another tool

[cmutel]

+1 to cruft, it is actually being maintained.