add Known Issues section to README

[aaronsteers]

RE: #329

This PR simply adds documentation for 2 common issues for users when they are getting started:

## Known Issues

* Multi-folder workspaces are not yet supported. ([info](https://github.com/hashicorp/vscode-terraform/issues/329#issuecomment-639378948))
* Single-folder workspaces in which the root module is not the top-level folder are not supported. ([info](https://github.com/hashicorp/vscode-terraform/issues/329#issuecomment-639107179))

[paultyng]

@aaronsteers this is our #1 priority at the moment, will merge this now, but we are hoping to have it resolved in the next week or two (see #423) so hopefully can modify it shortly afterwards.

[ghost]

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