WIP: add article documenting bundle and lookupFunction

Author: djbe

Documentation/Articles/Customise-loading-of-resources.md

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+# Customise loading of resources
+
+By default, the built-in templates provided by SwiftGen will load resources from the same bundle as where the generated code is located at:
+- If your generated code is in the main app, it will load resources from the `main` bundle.
+- If your generated code is in a Framework, it will load resources from that Framework's bundle.
+
+This works for most users, but there may be sitations where you'll need a more advanced solution.
+
+## Override the default Bundle
+
+Let's say you are developing a CocoaPods Pod for example. CocoaPods recommends the use of the `resource_bundles` option ([more info](https://guides.cocoapods.org/syntax/podspec.html#resource_bundles)) for adding resources to your Pod. Using this option, your resources will be grouped into a separate bundle.
+
+The advantage of this is that, no matter how an end-user adds your Pod to their project, your resources won't ever clash with their resources. This is especially important with static linking, where CocoaPods won't generate a Framework for each Pod and add everything to the main App bundle of a user.
+
+The disadvantage is that your resources are no longer located in the same bundle as your generated code, but in (for example) a sub-bundle `MyAwesomePodResources.bundle`. The generated code by SwiftGen will no longer work.
+
+### Solution
+
+To fix this, you can set the `bundle` template parameter to point to something in your code that represents your resources bundle.
+
+Let's say you have the following Swift code somewhere in your app:
+
+```swift
+final class MyAwesomePod {
+  // This is the bundle where your code resides in
+  static let bundle: Bundle = {
+    Bundle(for: MyAwesomePod.self)
+  }()
+
+  // Your resources bundle is inside that bundle
+  static let resourcesBundle: Bundle = {
+	guard let url = bundle.url(forResource: "MyAwesomePodResources", withExtension: "bundle"),
+      let bundle = Bundle(url: url) else {
+      fatalError("Can't find 'MyAwesomePodResources' bundle")
+    }
+    return bundle
+  }()
+}
+```
+
+You want SwiftGen to generate code to load resources from `MyAwesomePod.resourcesBundle`. Update your configuration file and add the `bundle` parameter, like so:
+
+```yaml
+input_dir: Resources
+output_dir: Sources
+strings:
+  inputs: en.lproj/Localizable.strings
+  outputs:
+    templateName: structured-swift5
+    output: Generated/Strings.swift
+    params:
+      bundle: MyAwesomePod.resourcesBundle
+```
+
+Run SwiftGen again to update the generated coide, and voila! Your generated code now works with a resources bundle.
+
+## Override the lookup function
+
+TODO