Add i18n instructions

I18N.md

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+# Internationalization / Localization
+
+We use transifex for localization.
+
+The process for localization is:
+
+ 1. Export localization source strings from Xcode
+
+     - Open `EduVPN.xcworkspace` in Xcode
+     - In Xcode, open the Projects and Targets pane
+        - In the Project Navigator (keyboard shortcut: Cmd+1), select "EduVPN" at the top left
+     - Click on _Product > Export Localizations_ 
+     - Select a location to save (say "eduVPN/i18n"; Xcode will create a directory with a bunch of files inside)
+     - In _Localizations_, ensure only _English - Default Localization_ is selected
+     - Click on the _Export_ button
+
+    The directory will now have an `en.xcloc` directory. Inside that, under `Localized Contents`, there will be an `en.xliff` file. This is our localization source.
+
+
+ 2. Import localization source strings into Transifex
+
+    When starting with a fresh app (like a new app or new redesign of an app):
+
+     - Log in to Transifex
+     - Under the _eduVPN apple (macOS/iOS)_ project, select _Resources_
+     - Click on _Add Resources_ to import the `en.xliff` file (use drag and drop -- the file picker doesn't work because it doesn't go inside the xcloc package)
+
+    When the app gets updated and the source strings change, we might want to update the strings in Transifex:
+
+     - Log in to Transifex
+     - Under the _eduVPN apple (macOS/iOS)_ project, select _Resources_
+     - Click on the `en.xliff` resource
+     - Click on _Update source file_ to import the `en.xliff` file (use drag and drop -- the file picker doesn't work because it doesn't go inside the xcloc package)
+
+ 3. Translate strings in Transifex
+
+ 4. Export localized strings from Transifex
+
+     - Log in to Transifex
+     - Under the _eduVPN apple (macOS/iOS)_ project, select _Resources_
+     - Click on the relevant `en.xliff` file
+     - Click on the language whose translations need to be imported
+     - Click on _Download for use_
+     - Save the `.xlf` file somewhere
+
+ 5. Import localized strings into Xcode
+
+     - Open `EduVPN.xcworkspace` in Xcode
+     - In Xcode, open the Projects and Targets pane
+        - In the Project Navigator (keyboard shortcut: Cmd+1), select "EduVPN" at the top left
+     - Click on _Product > Import Localizations_ 
+     - Select the `.xlf` file downloaded from Transifex
+     - Click on the _Import_ button
+     - If prompted on where we should import into, select `EduVPN.xcworkspace` (I don't think it matters what we choose here), click on _Choose_
+     - Xcode will show the currently used strings on the right and the being-imported strings on the left
+     - Click on _Import_
+     - Commit the changes
+