Easier access to opencv.js in tutorial

doc/js_tutorials/js_setup/js_setup/js_setup.markdown

 Build OpenCV.js {#tutorial_js_setup}
 ===============================
 
+@note
+You don't have to build your own copy if you simply want to start using it. Refer the Using Opencv.js tutorial for steps on getting a prebuilt copy from our releases or online documentation.
 
 Installing Emscripten
 -----------------------------

doc/js_tutorials/js_setup/js_usage/js_usage.markdown

@@ -4,7 +4,7 @@ Using OpenCV.js {#tutorial_js_usage}
 Steps
 -----
 
-In this tutorial, you will learn how to include and start to use `opencv.js` inside a web page.
+In this tutorial, you will learn how to include and start to use `opencv.js` inside a web page. You can get a copy of `opencv.js` from `opencv-{VERSION_NUMBER}-docs.zip` in each [release](https://github.com/opencv/opencv/releases), or simply download the prebuilt script from the online documentations at "https://docs.opencv.org/{VERISON_NUMBER}/opencv.js" (For example, [https://docs.opencv.org/3.4.0/opencv.js](https://docs.opencv.org/3.4.0/opencv.js). Use `master` if you want the latest build). You can also build your own copy by following the tutorial on Build Opencv.js.
 
 ### Create a web page
 
@@ -44,7 +44,7 @@ To run this web page, copy the content above and save to a local index.html file
 
 Set the URL of `opencv.js` to `src` attribute of \<script\> tag.
 
-@note For this tutorial, we host `opencv.js` at same folder as index.html.
+@note For this tutorial, we host `opencv.js` at same folder as index.html. You can also choose to use the URL of the prebuilt `opencv.js` in our online documentation.
 
 Example for synchronous loading:
 @code{.js}