Update README with warning about v3 breaking changes (#771)

Added important note about upcoming breaking changes in v3.

README.md

@@ -4,6 +4,9 @@
 
 # Mobile Verification Toolkit
 
+> [!IMPORTANT]  
+> Soon we will merge the v3 pull request which will result in breaking changes. If you rely on mvt output in other script make sure to the the branch before we merge. More details: https://github.com/mvt-project/mvt/issues/757
+
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