Add release notes

Author: IvanChepurnyi

RELEASE_NOTES.md

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+# Release 1.1.0
+New functionality:
+
+* Added normalizer for PSR-6 compatible key. `Psr6Normalizer` uses `EncodeNormalier` and `LengthNormalizer` behind the scenes. 
+
+# Release 1.0.0
+Initial features:
+
+* **Encode Normalizer** encodes all characters except alphanum into hex representation 
+* **Length Normalizer** strips characters after some length is reached 
+* **Scalar Converter** converts any non string scalar to string representation 
+* **Hash Converter** converts any array into md5 hash of json encoded value
+* **Key Value Converter** converts simple key value pair array into string representation like `key1_value1_key2_value2`