Improves instructions + adds marketplace badges

[bravo-kernel]

Refs https://github.com/ikappas/vscode-phpcs/issues/39, adds:

• fool/newbie proof instructions for testing the extension using the development branch
• marketplace badges (rating can be removed if you like)

The server side testing part in the readme is unchanged because I am unsure what it is used for.

[ikappas]

@bravo-kernel Thank you for this mr!