Add QA migration guide (#14544)

* Add QA migration guide

* Add full length gif of updating the build

* Use directory term where it seems more appropriate and better formatting

docs/QA_MIGRATIONS_GUIDE.md

@@ -0,0 +1,20 @@
+### QA Migrations Guide
+Migrations are needed to change top-level state data, this can be found in the browser's storage. This can look like removing specific keys/value pairs from state, changing objects to an array of objects, changing the name of a controller, etc.
+
+Steps
+  1. Create a new MetaMask directory\* folder locally with the source files before the migration, and load it as an unpacked extension in Chrome\*. If the migration is in a pull request, then build the `develop` branch to load. If the migration is already in `develop`, get a commit before the migration was added to build.
+      
+      ![Load unpacked extension to chrome](./assets/load-build-chrome.gif) 
+
+      #####  \* For migrations targeting specific features behind a feature flag add them appropriately to the `.metamaskrc` file before building. 
+      ##### \* In order for the "Load unpacked" button to be shown developer mode needs to be enabled.
+
+  2. Once the build has been loaded and state data has been initialized, ensure that the data in question that the migration targets is present in the local storage data.
+      
+      ![Chrome storage state](./assets/chrome-storage-local.png)
+
+  3. To trigger the migration a build with the migration will need to replace the files in the directory where the extension is loaded from, and refresh the extension.
+      
+      ![gif of replacing files and reloading the extension](./assets/folder-file-replacement-build.gif)
+  
+  4. Ensure that the data has been changed/deleted/etc.