DOC: update readme with link to new forum

[skip ci]

Author: j-bowhay

README.rst

@@ -27,7 +27,7 @@ ODE solvers, and more.
 - **Website:** https://scipy.org
 - **Documentation:** https://docs.scipy.org/doc/scipy/
 - **Development version of the documentation:** https://scipy.github.io/devdocs
-- **Mailing list:** https://mail.python.org/mailman3/lists/scipy-dev.python.org/
+- **SciPy development forum:** https://discuss.scientific-python.org/c/contributor/scipy 
 - **Source code:** https://github.com/scipy/scipy
 - **Contributing:** https://scipy.github.io/devdocs/dev/index.html
 - **Bug reports:** https://github.com/scipy/scipy/issues