Add example to README

Author: peterfication

README.md

@@ -2,6 +2,22 @@
 
 A CLI tool that can peek into `*.npy` files.
 
+## Example
+
+```bash
+$ num-peek assets/demo.npy
+Peek into assets/demo.npy
+----------------------------------------
+Dimensions: 2
+Shape: [2, 3]
+Type: Int8
+----------------------------------------
+Number of unique values: 6
+Unique values: [1, 3, 4, 8, 12, 22]
+Min value: 1
+Max value: 22
+```
+
 ## Development
 
 ### Prerequisites