Add usage to README

.gitignore

@@ -2,4 +2,5 @@
 *.jpg
 *.txt
 *.pdf
-.idea
+.idea
+cryptpng

README.md

@@ -1,3 +1,13 @@
-# cryptpng
+# Cryptpng
 
-A proof of concept implementation of storing encrypted data inside of png metadata chunks.
+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>
+```