Add link to migration guide

Signed-off-by: hossainemruz <hossainemruz@gmail.com>

README.md

@@ -1,3 +1,5 @@
+**If you are migrating from v3 (git submodule based) theme to v4 (hugo modules based) theme, please read this [migration guide](https://toha-guides.netlify.app/posts/update-v3-to-v4/).**
+
 # Toha
 
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