docs: update the README.md

[mato533]

Proposed changes

Update README.md

• add GIF image
• Indicate availability on VSCode Marketplace

Types of changes

• Polish (an improvement for an existing feature)
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (improvements to the project's docs)
• Internal updates (everything related to internal scripts, governance documentation and CI files)

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation (if appropriate)
• I have added proper type definitions for new commands (if appropriate)

Further comments

Reviewers: @webdriverio/project-committers

[mato533]

@christian-bromann
Explicitly stated in README.md that it is available in the VSCode Marketplace!

If there is a future request to make it available on OpenVSX, we can update this as well after it is released.