Reference to vscode-mta-tools

[piejanssens]

Referring to: https://github.com/cloudfoundry/multiapps-controller/wiki/Deployment-Descriptor#editor-schema-support

It might be beneficial for the users to refer to https://github.com/SAP/vscode-mta-tools and also update https://help.sap.com/docs/BTP/65de2977205c403bbc107264b8eccf4b/f48880b0295d4e9d859658244da84201.html#schema-store

It supports autocomplete and validation of mta and mtaext.