add installation manual

README.md

@@ -3,3 +3,14 @@
 <img src="https://img.icons8.com/clouds/100/000000/starburst-shape.png">
 
 Copy your selected text or page link in browser and turn it into markdown snippet!
+
+## Installation
+
++ Download the latest release [here](https://github.com/nettee/copy-it-as-markdown/releases), and unpack it.
++ Open _extensions_ page [chrome://extensions/](chrome://extensions/) in Chrome.
++ Toggle on **Developer mode** on top right.
++ Click **Load unpacked extension** and choose the unpacked directory.
+
+## Usage
+
+Right-click on your web page to use it!