golang: add README (#7207)

plugins/golang/README.md

+# Golang plugin
+
+This plugin adds completion for the [Go Programming Language](https://golang.org/),
+as well as some aliases for common Golang commands.
+
+To use it, add `golang` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... golang)
+```
+
+## Aliases
+
+| Alias   | Command                 | Description                                                   |
+| ------- | ----------------------- | ------------------------------------------------------------- |
+| gob     | `go build`              | Build your code                                               |
+| goc     | `go clean`              | Removes object files from package source directories          |
+| god     | `go doc`                | Prints documentation comments                                 |
+| gof     | `go fmt`                | Gofmt formats (aligns and indents) Go programs.               |
+| gofa    | `go fmt ./...`          | Run go fmt for all packages in current directory, recursively |
+| gog     | `go get`                | Downloads packages and then installs them to $GOPATH          |
+| goi     | `go install`            | Compiles and installs packages to $GOPATH                     |
+| gol     | `go list`               | Lists Go packages                                             |
+| gop     | `cd $GOPATH`            | Takes you to $GOPATH                                          |
+| gopb    | `cd $GOPATH/bin`        | Takes you to $GOPATH/bin                                      |
+| gops    | `cd $GOPATH/src`        | Takes you to $GOPATH/src                                      |
+| gor     | `go run`                | Compiles and runs your code                                   |
+| got     | `go test`               | Runs tests                                                    |
+| gov     | `go vet`                | Vet examines Go source code and reports suspicious constructs |