docs(Manual): add Update JCSUnity

Author: jcs090218

docs/Manual/Update JCSUnity.md

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+# Update JCSUnity
+
+We recommend you to delete the entire `JCSUnity` folder inside Unity editor.
+Unity usually works well by finding the correct component links between
+each assets.
+
+1. Open up your project with Unity
+2. Delete the entire `JCSUnity` folder (with Unity editor)
+3. Import the latest/targeted JCSUnity version
+
+You can find all our releases in our [release page](https://github.com/jcs090218/JCSUnity/releases).