SA-369 - Update developer documentation for multiplatform build

docs/developers/multiplatform.md

@@ -0,0 +1,32 @@
+# Multiplatform
+---
+## Install Homebrew
+https://brew.sh/
+
+## Install `xcodegen` from Homebrew
+```bash
+brew install xcodegen
+```
+
+## Create a `Local.xcconfig` file
+
+In the directory `{project root}/iosApp/` create a file named `Local.xcconfig` and populate it with 
+the following, removing the placeholders `<>` and the contents of the placeholders:
+
+_Note: Xcode treats `//` within a `.xcconfig` file as a comment, so in order to escape this, add `$()`
+between // . For example: `https:/$()/example.com`_
+
+```bash
+DEVELOPMENT_TEAM = <APPLE_DEVELOPMENT_TEAM_ID>
+TILE_PROVIDER_URL = https:/$()/tiles.example.com/
+SEARCH_PROVIDER_URL = https:/$()/search.example.com/
+EXTRACT_PROVIDER_URL = https:/$()/extracts.example.com/
+```
+You can find your `DEVELOPMENT_TEAM_ID` at https://developer.apple.com/
+
+### If using Android Studio
+
+- Build configurations should auto generate, and you can run the app on a **Simulator** / **Device** in the same way as you would an 
+Android App on an **Emulator** / **Device**.
+- Android Studio may notify you that the build process may be slower due to a missing guard, it may be 
+effective to implement the fix suggested.