Update contribution guidelines in README (#205)

Added a reference to the Python Developer's Guide for contribution guidelines.

Author: StanFromIreland

README.md

@@ -67,7 +67,8 @@ as if it was properly installed.
 
 # Contributions
 
-Contributions are welcome under all the same conditions as for CPython.
+Contributions are welcome under all the same conditions as for CPython, see
+the [Python Developer's Guide](https://devguide.python.org/) for more information.
 
 # Release Schedule
 
@@ -97,4 +98,4 @@ See the [LICENSE](https://github.com/python/pymanager/blob/main/LICENSE) for
 information on the terms & conditions for usage, and a DISCLAIMER OF ALL
 WARRANTIES.
 
-All trademarks referenced herein are property of their respective holders.
+All trademarks referenced herein are property of their respective holders.