Update the README file

Fix #441

Author: Minggang Wang

README.md

@@ -13,7 +13,7 @@ master | [![Build Status](https://travis-ci.org/RobotWebTools/rclnodejs.svg?bran
 
 1.Build from scratch.
 
-ROS is a cross-platform system, which covers Linux, macOS and Windows, and the `rclnodejs` module is developed on the [`master`](https://github.com/ros2/ros2/blob/master/ros2.repos) branch of ROS, so you have to build ROS from scratch at the present stage. Please select the platform you want to work on, then reference the instruction to build ROS.
+ROS is a cross-platform system, which covers Linux, macOS and Windows, and the `rclnodejs` module is developed on the [`master`](https://github.com/ros2/ros2/blob/master/ros2.repos) branch of ROS, so you have to build ROS from scratch at the present stage. Please select the platform you want to work on, then reference the instruction to build ROS (please build wiht flag `--merge-install`).
 
 * [Linux](https://github.com/ros2/ros2/wiki/Linux-Development-Setup)
 * [macOS](https://github.com/ros2/ros2/wiki/OSX-Development-Setup)
@@ -85,6 +85,22 @@ npm install
   set PATH=<path\to\python 2.x>;%PATH%
 ```
 
+## Install rclnodejs by npm
+
+### Install from npmjs
+
+Please run `npm i rclnodjs` to install the latest version OR `npm i rclnodjs@<version>` to install a specific version.
+
+### Install from GitHub
+
+You can also install `rclnodejs` from Github to experience the latest features, please add the following line into the `dependencies` section of your `package.json` file.
+
+```"rclnodejs": "RobotWebTools/rclnodejs"```
+
+We don't offer a release of rclnodejs against a specific release of ROS 2, e.g. [bouncy-bolson](https://github.com/ros2/ros2/releases/tag/release-bouncy), as the one pubilshed on npmjs is always built against the master branch of ROS 2, but we do have the branch which is against a release of ROS 2 on GitHub. You can checkout the [branches](https://github.com/RobotWebTools/rclnodejs/branches) we have, then add it to your `package.json`, e.g. if you want to use ROS 2 Bouncy Bolson:
+
+```"rclnodejs": "RobotWebTools/rclnodejs#bouncy-bolson"```
+
 ## Run Unit Test
 
 [mocha](https://www.npmjs.com/package/mocha) is a javascript test framework for node.js, simply run the following command to run the unit test under `test` folder:
@@ -128,9 +144,9 @@ There are also several useful examples under the `example` folder, which will sh
 
 The API spec is generated by `jsdoc`, you can manually run `npm run docs` to create them by yourself, or just use the existing documents under `docs` folder. To visit the on-line version, please navigate to http://robotwebtools.org/rclnodejs/docs/index.html in your browser.
 
-## Experimental
+## Experimental - Deprecated
 
-* [actionlib](https://github.com/RobotWebTools/rclnodejs/blob/develop/tutorials/actionlib.md)
+* [actionlib](https://github.com/RobotWebTools/rclnodejs/blob/develop/tutorials/actionlib.md) - as the [rcl](https://github.com/ros2/rcl) library has implemented the action related functions, we are going to drop this one and we don't garantee the current actionlib can work with [rclcpp](https://github.com/ros2/rclcpp).
 
 ## Get Involved