Added a note about performance (#67)

Co-authored-by: Jordi Boggiano <j.boggiano@seld.be>

Author: staabm

README.md

@@ -47,6 +47,11 @@ try {
 }
 ```
 
+> **Note:** This library is meant to parse JSON while providing good error messages on failure. There is no way it can be as fast as php native `json_decode()`.
+>
+> It is recommended to parse with `json_decode`, and when it fails parse again with seld/jsonlint to get a proper error message back to the user. See for example [how Composer uses this library](https://github.com/composer/composer/blob/56edd53046fd697d32b2fd2fbaf45af5d7951671/src/Composer/Json/JsonFile.php#L283-L318):
+
+
 Installation
 ------------