[DOCS] Add README

Author: nhovratov

README.md

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+# Mask to Content Blocks migration
+
+This TYPO3 extension helps you migrate your Mask elements on TYPO3 v13 to TYPO3 CMS Content Blocks, the official TYPO3
+extension to define Content Types.
+
+You need a running TYPO3 instance with your loaded Mask elements.
+
+Install this extension via composer:
+
+```
+composer req nhovratov/mask-to-content-blocks
+```
+
+Then run the migration command. This migration will create the Content Blocks into the same extension, where your Mask
+elements are currently loaded.
+
+```
+bin/typo3 mask-to-content-blocks:migrate
+```
+
+Next, remove Mask and this extension:
+
+```
+composer remove mask/mask nhovratov/mask-to-content-blocks
+```
+
+## Final steps
+
+This migration command is no guarantee that everything will work perfectly. Check the generated Content Blocks by
+yourself and see, if everything is fine. Also, any TypoScript overrides need migration from `lib.maskElement` to
+`lib.contentBlock`. Test your frontend template and backend preview. They might need slight adjustments.
+
+For more information, visit the manual migration guide in the Content Blocks documentation: https://docs.typo3.org/permalink/friendsoftypo3-content-blocks:migrations-mask