Add libudev to installation readme

Author: Nir-Az

doc/installation.md

@@ -37,7 +37,8 @@ The scripts and commands below invoke `wget, git, add-apt-repository` which may
      Unplug any connected Intel RealSense camera.<br />  
 
   2. Install the core packages required to build *librealsense* binaries and the affected kernel modules:  
-    `sudo apt-get install git libssl-dev libusb-1.0-0-dev pkg-config libgtk-3-dev`  <br /><br />
+
+    `sudo apt-get install git libssl-dev libusb-1.0-0-dev libudev-dev pkg-config libgtk-3-dev`  <br /><br />
     Distribution-specific packages:  <br />
      * Ubuntu 14 or when running Ubuntu 16.04 live-disk:<br />
       `./scripts/install_glfw3.sh`  <br />
@@ -51,8 +52,11 @@ The scripts and commands below invoke `wget, git, add-apt-repository` which may
 > **Cmake Note**: certain librealsense CMAKE flags (e.g. CUDA) require version 3.8+ which is currently not made available via apt manager for Ubuntu LTS.   
     Go to the [official CMake site](https://cmake.org/download/) to download and install the application  
 
-  **Note** on graphic sub-system utilization:<br />
-     *glfw3*, *mesa* and *gtk* packages are required if you plan to build the SDK's OpenGL-enabled examples. The *librealsense* core library and a range of demos/tools are designed for headless environment deployment.
+  **Note**: 
+
+* on graphic sub-system utilization:<br />
+       *glfw3*, *mesa* and *gtk* packages are required if you plan to build the SDK's OpenGL-enabled examples. The *librealsense* core library and a range of demos/tools are designed for headless environment deployment.
+* `libudev-dev` installation is optional but recommended, when the `libudev-dev` is installed the SDK will use an event driven approach for triggering USB detection and enumeration, if not the SDK will use a timer polling approach which is less sensitive for device detection.
 
   3. Run Intel Realsense permissions script from librealsense root directory:<br />
     `./scripts/setup_udev_rules.sh`  
@@ -61,15 +65,14 @@ The scripts and commands below invoke `wget, git, add-apt-repository` which may
     *`./scripts/setup_udev_rules.sh --uninstall`*
 
   4. Build and apply patched kernel modules for: <br />
-    * **Ubuntu 14/16/18 with LTS kernel**
 
+    * **Ubuntu 14/16/18 with LTS kernel**
       `./scripts/patch-realsense-ubuntu-lts.sh`<br />
     * **Ubuntu with Kernel 4.16**
-
       `./scripts/patch-ubuntu-kernel-4.16.sh`<br />  
     * **Intel® Joule™ with Ubuntu**
       Based on the custom kernel provided by Canonical Ltd.  
-
+    
       `./scripts/patch-realsense-ubuntu-xenial-joule.sh`<br />
 
       The script above will download, patch and build realsense-affected kernel modules (drivers).<br />
@@ -122,15 +125,15 @@ The scripts and commands below invoke `wget, git, add-apt-repository` which may
     * `cmake ../ -DBUILD_EXAMPLES=true -DBUILD_GRAPHICAL_EXAMPLES=false` - For systems without OpenGL or X11 build only textual examples<br /><br />
 
   * Recompile and install *librealsense* binaries:<br />  
-  `sudo make uninstall && make clean && make && sudo make install`<br />  
-  The shared object will be installed in `/usr/local/lib`, header files in `/usr/local/include`.<br />
-  The binary demos, tutorials and test files will be copied into `/usr/local/bin`<br />
-  **Tip:** Use *`make -jX`* for parallel compilation, where *`X`* stands for the number of CPU cores available:<br />
-  `sudo make uninstall && make clean && make **-j8** && sudo make install`<br />
-  This enhancement may significantly improve the build time. The side-effect, however, is that it may cause a low-end platform to hang randomly.<br />
-  **Note:** Linux build configuration is presently configured to use the V4L2 backend by default.<br />
-  **Note:** If you encounter the following error during compilation `gcc: internal compiler error` it might indicate that you do not have enough memory or swap space on your machine. Try closing memory consuming applications, and if you are running inside a VM, increase available RAM to at least 2 GB.<br />
-  **Note:** You can find more information about the available configuration options on [this wiki page](https://github.com/IntelRealSense/librealsense/wiki/Build-Configuration).
+    `sudo make uninstall && make clean && make && sudo make install`<br />  
+    The shared object will be installed in `/usr/local/lib`, header files in `/usr/local/include`.<br />
+    The binary demos, tutorials and test files will be copied into `/usr/local/bin`<br />
+    **Tip:** Use *`make -jX`* for parallel compilation, where *`X`* stands for the number of CPU cores available:<br />
+    `sudo make uninstall && make clean && make **-j8** && sudo make install`<br />
+    This enhancement may significantly improve the build time. The side-effect, however, is that it may cause a low-end platform to hang randomly.<br />
+    **Note:** Linux build configuration is presently configured to use the V4L2 backend by default.<br />
+    **Note:** If you encounter the following error during compilation `gcc: internal compiler error` it might indicate that you do not have enough memory or swap space on your machine. Try closing memory consuming applications, and if you are running inside a VM, increase available RAM to at least 2 GB.<br />
+    **Note:** You can find more information about the available configuration options on [this wiki page](https://github.com/IntelRealSense/librealsense/wiki/Build-Configuration).
 
   2. Install IDE (Optional):
     We use QtCreator as an IDE for Linux development on Ubuntu. Follow the  [link](https://wiki.qt.io/Install_Qt_5_on_Ubuntu) for QtCreator5 installation