add instructions

Author: wenliangyeoh

README.md

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+# A Grayscale Image Processing Widget
+
+GUI to explore OpenCV grayscale image processing functions.
+
+## Installation
+
+1. From packaged .exe file (Windows)
+    - Download latest .exe file from releases.
+    - Run .exe file
+2. From wheel
+    - Download latest wheel file from releases.
+    - Install wheel `pip install <package-name>.whl`
+    - Run widget `grayscale_image_processing`
+
+## Usage
+
+Drag and drop image onto widget.
+![Screenshot 1](/screenshots/screenshot1.png?raw=true "Screenshot 1")
+
+Select process and adjust parameters.
+![Screenshot 2](/screenshots/screenshot2.png?raw=true "Screenshot 2")
+![Screenshot 3](/screenshots/screenshot3.png?raw=true "Screenshot 3")