Add embind example to Bazel docs (#910)

* Add embind example to Bazel docs

* address feedback

bazel/test_external/BUILD

@@ -10,3 +10,22 @@ wasm_cc_binary(
     cc_target = ":hello-world",
 )
 
+cc_binary(
+    name = "hello-embind",
+    srcs = ["hello-embind.cc"],
+    # It's probably a good idea to use select() and config_setting
+    # to add different linkopts to BASE_LINKOPTS depending on things
+    # like a "opt" or "dbg" compilation mode.
+    linkopts = [
+        "--bind",       # Enable embind
+        "-sMODULARIZE",
+    ],
+    # This target won't build successfully on its own because of missing emscripten
+    # headers etc. Therefore, we hide it from wildcards.
+    tags = ["manual"],
+)
+
+wasm_cc_binary(
+    name = "hello-embind-wasm",
+    cc_target = ":hello-embind",
+)