Fix contributing broken link

Author: Theo Dias

README.rst

@@ -114,7 +114,7 @@ Contributing
 We welcome bug reports and fixes and improvements to the documentation.
 
 For more information on contributing, please see the
-`contributing guide <https://github.com/pymc-devs/pytensor/CONTRIBUTING.md>`__.
+`contributing guide <https://pytensor.readthedocs.io/en/latest/dev_start_guide.html>`__.
 
 A good place to start contributing is by looking through the issues
 `here <https://github.com/pymc-devs/pytensor/issues>`__.