Add documentation around usage of external code

[gep13]

As mentioned in this PR:

https://github.com/gep13/chocolatey-vscode/pull/120

[gep13]

Also, need to mention the usage of code from this repository:

https://github.com/tintoy/msbuild-project-tools-server

and this one:

https://github.com/tintoy/msbuild-project-tools-vscode

As these are being consulted to setup the configuration handler for this project.

Thanks @tintoy! 😄

[gep13]

Also making use of these:

https://github.com/Jason-Rev/vscode-spell-checker

https://github.com/DavidAnson/markdownlint