chore: Swift usage documentation (#3039)

Added documentation on how to use swift in the framework.

Co-authored-by: Philipp Hofmann <philipp.hofmann@sentry.io>

develop-docs/README.md

@@ -2,6 +2,10 @@
 
 This page contains internal documentation for development.
 
+## Coding with Swift
+
+To use Swift in the project take a look at [Swift Usage](Swift-Usage.md) documentation.
+
 ## Code Signing
 
 This repository follows the [codesiging.guide](https://codesigning.guide/) in combination with [fastlane match](https://docs.fastlane.tools/actions/match/).

develop-docs/Swift-Usage.md

@@ -0,0 +1,23 @@
+# Swift Usage
+
+Starting from version 8.0.0, it is now possible to include Swift code in the project. All Swift files should be placed under the `/Sources/Swift` directory.
+
+> In this document, `SentryPrivate` refers to the library written in Swift, while `Sentry` represents the framework written in Objective-C. The term `SentryPrivate` public API refers to the API that will be consumed by `Sentry` and is not intended for direct use by users.
+
+When working with Swift, it's important to keep the following restrictions in mind:
+
+1. All Swift code will be bundled within the `SentryPrivate` library, which `Sentry` depends on.
+2. User-facing APIs cannot be written in Swift because their components will be accessed through imports from "SentryPrivate."
+3. `SentryPrivate` does not have access to `Sentry` classes to avoid cyclic references. As a result, any code written in Objective-C is not accessible from the  Swift layer.
+    - However, it is possible to create Dependency Injection (DI) APIs in Swift, allowing `Sentry` to inject its objects for use within `SentryPrivate`.
+4. `SentryPrivate` public APIs (code consumed by `Sentry`) cannot utilize certain Objective-C incompatible features, including:
+    - Generics
+    - Non-@objc protocols and protocol extensions
+    - Top-level functions and properties
+    - Global variables
+    - Structs
+    - Swift-only enums
+    - Swift-only optionals
+    - Swift-only tuples
+
+By keeping these considerations in mind, you can effectively work with Swift code within the project, ensuring compatibility with the `Sentry` framework and adhering to the necessary restrictions imposed by the language differences.