Add link to upgrade guide to README
Also removes the link to discourse
- [Website](https://allennlp.org/)
- [Guide](https://guide.allennlp.org/)
- [Documentation](https://docs.allennlp.org/) ( [latest](https://docs.allennlp.org/latest/) | [stable](https://docs.allennlp.org/stable/) | [commit](https://docs.allennlp.org/main/) )
-- [Forum](https://discourse.allennlp.org)
+- [Upgrade Guide from 1.x to 2.0](https://github.com/allenai/allennlp/discussions/4933)
- [Stack Overflow](https://stackoverflow.com/questions/tagged/allennlp)
- [Contributing Guidelines](CONTRIBUTING.md)
- [Officially Supported Models](https://github.com/allenai/allennlp-models)
