codespaces: add documentation for configuring Go

[stamblerre]

See the discussion on microsoft/vscode-go#2977, and specifically @hyangah's comment: microsoft/vscode-go#2977 (comment).

[hyangah]

Many things had changed since then. online.visualstudio.com is moved to github codespaces, and vscode dev container team made significant progress wrt the container configuration.

I just checked github codespaces, and no extra configuration was necessary for Go.
The GitHub codespaces documentation still doesn't list Go, so I am not sure what's the current official status.

@Chuxel can I assume that GitHub codespaces supports Go environment?

[Chuxel]

@hyangah Yes, the default image now includes the latest versions of Go, Rust, and Ruby in addition to the platforms listed there. We'll generally keep the latest version of Go in the image as it releases. People can also use the Go dev container if they prefer to access older ones and we plan to add the same iteration process that exists for Remote - Containers into Codespaces (it's just a bit tricky to pull off).

We're also going to update default codespaces image to Ubuntu 20.04 and it should have the same base VS Code settings as the Go definition does. That part is in progress, but Go is there in the current image as well.

[hyangah]

Thanks! @Chuxel
I see Go is now listed in the repo README.md. Sent github/docs#2007 for github docs update.

I will close this issue.