Created a changelog of the changes

Author: lordthorzonus

CHANGELOG.md

@@ -0,0 +1,14 @@
+# Changelog
+
+## Updating from 1.0.0 to 2.0.0
+ - Overall php version requirement was bumped from `5.5` to `7.3`
+ - Fluent interfaces in `Dataloader::prime()`,`DataLoader::clear` and `DataLoader::clearAll()` were removed. So change usages like:
+    ```php
+    $dataloader->clear('A')->prime('A', 'Y');
+    ```
+    to:
+    ```
+    $dataloader->clear('A');
+    $dataloader->prime('A', 'Y');
+    ```
+ - All classes in the library have been marked as final.