Fix outdated and broken links in "Places to chat section" of Readme

[scottscheper]

The Places to chat section in the Solid Readme needs the following fixes:

1. This link (https://forum.solidproject.org/t/welcome-to-the-solid-forum-please-introduce-yourself-here-great-to-have-you-on-board/440/20) hotlinks to an irrelevant comment in the post (/20).
2. Same with this one: This link (https://forum.solidproject.org/t/simply-chat-a-place-for-light-hearted-quick-questions-and-comments/441/3) hotlinks to an irrelevant comment in the post (/3).
3. This link is broken: https://forum.solidproject.org/c/app-communities and results in a 404

Furthermore, the section should be updated to reference the Solid Gitter Chat, as well.