Improve readme

Author: rubencaro

README.md

@@ -35,6 +35,13 @@ Now you can use bare `encrypt/1` and `decrypt/1`:
 |> Cipher.decrypt  # "secret"
 ```
 
+### Decryption errors
+
+When you decrypt non-valid strings you can get two kinds of errors:
+
+* `{:error, "Could not decode string 'yourstring'"}` if your string was tampered or wrongly transferred.
+* `{:error, "Could not decrypt string 'yourstring'"}` if your string was encrypted using different keys. Maybe some edge cases of tampering too.
+
 ## Cipher/Parse JSON
 
 `cipher/1` and `parse/1`. Just as `encrypt/1` and `decrypt/1` but for JSON.