Avoid duplication of documentation between extension and wiki

[datho7561]

See PR #327

The markdown documentation for settings that is embedded in the extension is copied directly from the vscode-xml wiki pages. Come up with some mechanism to avoid maintaining two copies of the settings documentation.

[angelozerr]

It should be nice in the same PR to update or add links in the README.md of vscode-xml settings section to the docs md files https://github.com/redhat-developer/vscode-xml#supported-vs-code-settings