Update README.md to clarify that vscode-codeql also installs 'Test Adapter Converter' and 'Test Explorer UI'

[darkred]

This is a follow-up of my suggestion in #1542 (comment) (cc @aeisenberg).
It adds a clarification in README.md that two additional extensions are installed, being dependencies required for proper functionality.
This PR changes both README.md files, as my suggestion is to also include this clarification in the Marketplace page, as well as in the 'Details' tab of the VSCode's Extensions panel.

The markdown changes rendered:

Repo README.md

Marketplace README.md

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[aeisenberg]

Thanks for the PR. I'm going to do a little copy-editing here and then merge.