Add embind example to Bazel docs (#910)

* Add embind example to Bazel docs

* address feedback

bazel/test_external/hello-embind.cc

@@ -0,0 +1,16 @@
+#include <emscripten/bind.h>
+
+using namespace emscripten;
+
+class HelloClass {
+    public:
+    static std::string SayHello(const std::string &name) {
+        return "Yo! " + name;
+    };
+};
+
+EMSCRIPTEN_BINDINGS(Hello) {
+    emscripten::class_<HelloClass>("HelloClass")
+        .constructor<>()
+        .class_function("SayHello", &HelloClass::SayHello);
+}