docs: give a small example how JS<->wasm bridging is done

docs/index.rst

@@ -15,5 +15,6 @@ Contents:
    docker
    targets
    microcontrollers
+   webassembly
    faq
    internals

docs/webassembly.rst

@@ -0,0 +1,47 @@
+.. _webassembly:
+
+
+WebAssembly
+===========
+
+.. highlight:: go
+
+You can call a JavaScript function from Go and call a Go function from WebAssembly::
+
+    package main
+
+    // This calls a JS function from Go.
+    func main() {
+        println("adding two numbers:", add(2, 3)) // expecting 5
+    }
+
+    // This function is imported from JavaScript, as it doesn't define a body.
+    // You should define a function named 'main.add' in the WebAssembly 'env'
+    // module from JavaScript.
+    func add(x, y int)
+
+    // This function is exported to JavaScript, so can be called using
+    // exports.add() in JavaScript.
+    //go:export multiply
+    func multiply(x, y int) int {
+        return x * y;
+    }
+
+
+.. highlight:: javascript
+
+Related JavaScript would look something like this::
+
+    // Providing the environment object, used in WebAssembly.instantiateStreaming.
+    env: {
+        'main.add': function(x, y) {
+            return x + y
+        }
+        // ... other functions
+    }
+
+    // Calling the multiply function:
+    console.log('multiplied two numbers:', wasm.exports.multiply(5, 3));
+
+A more complete example is provided in the `wasm example
+<https://github.com/aykevl/tinygo/tree/master/src/examples/wasm>`_.