Add "Open with Codeanywhere" badge to .devcontainer/README.md

[nkkko]

Why?

With Codeanywhere, devs and contributors can:

• Instantly launch a cloud or local IDE with remote dev environment.
• Automatically set up and configure a remote dev environments if a devcontainer.json file is present.

Benefits:

1. Faster onboarding for new contributors.
2. Consistent and ready-to-use development environments.
3. Flexibility to work in the cloud or locally.

[vercel]

@nkkko is attempting to deploy a commit to the LlamaIndex Team on Vercel.

A member of the Team first needs to authorize it.