Add Extensions.json and readme guidance for VSCode

[swernli]

We should add a .vscode/extensions.json file to help recommend extensions for this repo. Also, while the Ionide extension is helpful for processing F# projects and the Omnisharp extension is recommended for C# projects, they have trouble playing well together. It took me a while to figure out how to get Ionide not to show errors for the C# projects that it fails to load and on the flip side how to avoid seeing thousands of errors registered by Omnisharp because it didn't understand namespaces that were coming from F# projects. I think we should add some additional guidance to the readme.md for vscode users to help with this set up.

Originally posted by @swernli in #371

[bettinaheim]

Sounds good. Since you've dealt with this, do you want to make a stab at it, @swernli?

[swernli]

I can definitely do that!