introduce slack channel for community (#6717)

Summary:

To better connect with our user as well as our community contributors, this diff introduces slack channels for people having broad questions as well as any interest in contributing directly.

Differential Revision: D65609750

Author: Gasoonjia

README.md

@@ -43,6 +43,11 @@ We recommend using the latest release tag from the
 See [CONTRIBUTING.md](CONTRIBUTING.md) for details about issues, PRs, code
 style, CI jobs, and other development topics.
 
+To connect with us and other community members, we invite you to join PyTorch Slack community by filling out this [form](https://docs.google.com/forms/d/e/1FAIpQLSeADnUNW36fjKjYzyHDOzEB_abKQE9b6gqqW9NXse6O0MWh0A/viewform). Once you've joined, you can:
+* Head to the `#executorch-general` channel for general questions, discussion, and community support.
+* Join the `#executorch-contributors` channel if you're interested in contributing directly to project development.
+
+
 ## Directory Structure
 
 ```