Add usage to README

.gitignore

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 *.txt
 *.pdf
 .idea
+cryptpng

README.md

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-# cryptpng
+# Cryptpng
 
 A proof of concept implementation of storing encrypted data inside of png metadata chunks.
+
+## Usage
+
+```shell script
+# encrypt
+cryptpng --image <name of the image> --in <input file> --out <output file>
+
+# decrypt
+cryptpng --decrypt --image <crypt image> --out <decrypted output file>
+```