Flutter extension tasks in pipelines yamp not validating in VSCode

[maks]

I thought it maybe useful to document in the Readme for those that may use VSCode to edit their azure-pipelines.yml file with Flutter tasks from this extension will find that they are marked as invalid because they are not contained "in-box" schema.

This answer on SO shows how to get auto-completion to work for the custom Flutter tasks, simply using the vss-extension.json from this repo as the custom json schema file as per the SO answer.