Document WEB_CONCURRENCY for Python

[sc-david-voisin]

As we have per Ruby and PHP, can we have a documentation about WEB_CONCURRENCY for Python.

https://doc.scalingo.com/languages/ruby/start#web_concurrency
https://doc.scalingo.com/languages/php/start#php-fpm-concurrency

[sc-david-voisin]

Also second remark from the customer, put the link to:
https://github.com/Scalingo/python-buildpack/blob/master/vendor/WEB_CONCURRENCY.sh

that is the source of truth.