Added Tidelift badge and sponsor link (#448)

Author: agronholm

.github/FUNDING.yml

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+tidelift: pypi/sqlacodegen

README.rst

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 .. image:: https://coveralls.io/repos/github/agronholm/sqlacodegen/badge.svg?branch=master
   :target: https://coveralls.io/github/agronholm/sqlacodegen?branch=master
   :alt: Code Coverage
+.. image:: https://tidelift.com/badges/package/pypi/sqlacodegen
+  :target: https://tidelift.com/subscription/pkg/pypi-sqlacodegen
+  :alt: Tidelift
 
 This is a tool that reads the structure of an existing database and generates the
 appropriate SQLAlchemy model code, using the declarative style if possible.