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

[nkkko]

Description

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.

All Promptflow Contribution checklist:

• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.
• I confirm that all new dependencies are compatible with the MIT license.
• Create an issue and link to the pull request to get dedicated review from promptflow team. Learn more: suggested workflow.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.