Update README.md

README.md

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 **WARNING: Latest `raygui` from master branch is always aligned with latest `raylib` from master branch. Make sure to use the appropiate versions.**
 
+**WARNING: Master branch now contains raygui 4.0-dev, it is an API-BREAKING reddesign from previous versions (3.x), now all functions are more consistent and coherent, you can read the details about this breaking change in issue [283](https://github.com/raysan5/raygui/issues/283)**
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 ## features
 
  - **Immediate-mode gui, no retained data**