Add docstring for the ChaCha20 algorithm

Author: Susmita-Chakrabarty

crypto/ChaCha20.py

@@ -0,0 +1,17 @@
+from typing import List
+import struct
+
+__all__ = ['ChaCha20']
+class ChaCha20:
+    """
+    Implementation of the ChaCha20 stream cipher.
+    
+    Attributes
+    ----------
+    key : bytes
+        32-byte (256-bit) encryption key.
+    nonce : bytes
+        12-byte (96-bit) nonce.
+    counter : int
+        32-bit counter, typically starts at 0.
+    """