Docs: Add migrating instructions for macOS using AppleScript

doc/migrate-osx.md

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+macOS Migration Instructions and Notes
+====================================
+
+Motivation
+-----------
+This guide is addressing the issue of increasing RavenCore-QT.app (to be exact - Application Support directory) size when installed on mac's internal storage. 
+
+The outcome of this guide is to help with safely transferring the Application Support directory and the app itself to an external storage.
+
+Please use this guide if you have already installed Raven Wallet before using [this guide](https://github.com/RavenProject/Ravencoin/blob/master/doc/build-osx.md)
+
+Pre-Requisites
+-----------
+1. Your RavenCore-QT.app is installed in internal storage
+2. You have found the Application Support directory path ([navigate to this link](https://github.com/RavenProject/Ravencoin/blob/master/doc/build-osx.md#running) to refresh your memory)
+3. You have backed-up all the data from wallet (12-phrase, etc.)
+4. Your external storage has APFS format (no tests have been conducted for other file formats like FAT, NTFS, EXT, etc.)
+5. The external storage should be at least 100+ GiB
+
+Guide
+-----------
+Connect your external storage to your mac. Confirm that the OS has detected the storage
+
+### Transfer RavenCore-QT.App and Application Support Directory
+1. Open Finder and navigate to the Application Folder
+2. Drag and Drop Raven App to your external storage
+3. Navigate to your Application Support directory
+4. Copy the Application Support directory to the external storage
+
+### AppleScript
+1. Open Spotlight search
+2. Find and open AppleScript Editor(or Script Editor)
+3. In the menu bar find "File"
+4. Click on "New"
+5. In the new file editor insert
+```
+do shell script "open -a '/Volumes/<Your_External_Storage_Name>/Raven-Qt.app' --args -datadir=/Volumes/<Your_External_Storage_Name>/Application\ Support"
+```
+Note: this example above is assuming your have copied App and support directory to the root path of your external storage. Edit the path if your path is different
+
+6. Save the script (In menu bar File->Save)
+7. Export the script to app (Export->File Format: App)
+8. Pin your new app to the dock
+9. Click on the app you have just created using AppleScript to verify that your RavenWallet is working properly as before
+
+### Finish
+If you have successully achieved all steps above, you can remove the RavenCore-QT.app in your internal storage -> Applications folder and internal storage -> Application Support Directory
+
+<b>Please don't do it</b> if you have had issues along the way. Open an issue for the community's help
+
+Notes
+-----
+
+* Tested on OS X 10.15.7 on 64-bit Intel processors only.
+
+* Tested on 1 TiB NVMe external storage formated to APFS