Add Go as an embedding to the book (#1481)

* Add Go as an embedding to the book

Also take this time to list out all embeddings in the README of wasmtime
itself.

docs/lang-go.md

@@ -0,0 +1,76 @@
+# Go
+
+Wasmtime [is available as a Go
+Module](https://pkg.go.dev/github.com/bytecodealliance/wasmtime-go). This guide
+will go over adding Wasmtime to your project, and some provided examples of what
+can be done with WebAssembly modules.
+
+Make sure you're using Go 1.12 or later with modules support.
+
+## Getting started and simple example
+
+First up you'll want to start a new module:
+
+```sh
+$ mkdir hello-wasm
+$ cd hello-wasm
+$ go mod init hello-wasm
+```
+
+Next, copy this example WebAssembly text module into your project. It exports a
+function for calculating the greatest common denominator of two numbers.
+
+```wat
+{{#include ../examples/gcd.wat}}
+```
+
+Next, we can write our code in `main.go` which reads this file and runs it:
+
+```go
+package main
+
+import (
+    "fmt"
+    "github.com/bytecodealliance/wasmtime-go"
+)
+
+func main() {
+    store := wasmtime.NewStore(wasmtime.NewEngine())
+    module, err := wasmtime.NewModuleFromFile(store, "gcd.wat")
+    check(err)
+    instance, err := wasmtime.NewInstance(module, []*wasmtime.Extern{})
+    check(err)
+
+    gcd := instance.GetExport("gcd").Func()
+    val, err := gcd.Call(6, 27)
+    check(err)
+    fmt.Printf("gcd(6, 27) = %d\n", val.(int32))
+}
+
+func check(err error) {
+    if err != nil {
+        panic(err)
+    }
+}
+```
+
+And finally we can build and run it:
+
+```sh
+$ go run main.go
+gcd(6, 27) = 3
+```
+
+If this is the output you see, congrats! You've successfully ran your first
+WebAssembly code in Go!
+
+## More examples and contributing
+
+The `wasmtime` Go package [lives in its own
+repository](https://github.com/bytecodealliance/wasmtime-go) and has a [number
+of other more advanced
+examples](https://pkg.go.dev/github.com/bytecodealliance/wasmtime-go?tab=doc#pkg-examples)
+as well. Feel free to browse those, but if you find anything missing don't
+hesitate to [open an
+issue](https://github.com/bytecodealliance/wasmtime-go/issues/new) and let us
+know if you have any questions!