Update README.md to add a description of how to install the pre-built engine library (sony#17)

Add description how to install the pre-built engine library

Author: Hidenori Matsubayashi

README.md

@@ -98,7 +98,7 @@ $ sudo apt install libdrm-dev libgbm-dev libinput-dev libudev-dev libsystemd-dev
 
 This embedder requres `libflutter_engine.so` (Flutter embedder library). You need to install `libflutter_engine.so` to `/usr/lib`. See: [Building Flutter Engine embedder](./BUILDING-ENGINE-EMBEDDER.md)
 
-Or you can download a specific Flutter Engine by the following steps, but it's limited to debug mode and x64 targets.
+Or you can download a specific pre-built Flutter Engine from Google's infra by the following steps, but it's limited to **debug mode** and **x64** targets.
 
 [Step1] Check the version (SHA) of the channel you want to use.
 - [master channel](https://raw.githubusercontent.com/flutter/flutter/master/bin/internal/engine.version)
@@ -111,6 +111,12 @@ Or you can download a specific Flutter Engine by the following steps, but it's l
 $ curl -O https://storage.googleapis.com/flutter_infra/flutter/FLUTTER_ENGINE/linux-x64/linux-x64-embedder
 ```
 
+[Step3] Install the library. Note that the downloaded library is only **debug mode** and for **x64** targets.
+```Shell
+$ unzip ./linux-x64-embedder
+$ sudo cp ./libflutter_engine.so /usr/lib
+```
+
 ## 2. Examples
 There are sample projects in [`examples`](./examples) directory. You can also comunicate with Dart code by using the plugin APIs with the same specifications as with Flutter desktops for Windows.