add readme

Author: fumito-ito

README.md

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+
+[![Swift](https://github.com/fumito-ito/FirebaseRemoteConfig-OpenFeature-Provider-Swift/actions/workflows/swift.yml/badge.svg?branch=main)](https://github.com/fumito-ito/FirebaseRemoteConfig-OpenFeature-Provider-Swift/actions/workflows/swift.yml)
+
+# FirebaseRemoteConfig OpenFeature Provider for Swift
+
+This is yet another OpenFeature provider for [Firebase RemoteConfig](https://firebase.google.com/docs/remote-config?hl=en).
+
+## Installation
+
+### Swift Package Manager
+
+In dependencies section of Package.swift add:
+
+```swift
+dependencies: [
+    .package(
+        url: "[email protected]:fumito-ito/FirebaseRemoteConfig-OpenFeature-Provider-Swift.git",
+        .upToNextMajor(from: "0.0.1")
+    ),
+]
+```
+
+and in the target dependencies section add:
+
+```swift
+.product(name: "FirebaseRemoteConfigOpenFeatureProvider", package: "FirebaseRemoteConfig-OpenFeature-Provider-Swift"),
+```
+
+## Usage
+
+Import the `FirebaseRemoteConfigOpenFeatureProvider` and `OpenFeature` modules.
+
+```swift
+import FirebaseRemoteConfigOpenFeatureProvider
+```
+
+Create and set provider.
+
+```swift
+let provider = FirebaseRemoteConfigOpenFeatureProvider(remoteConfig: RemoteConfig.remoteConfig())
+let context = MutableContext(targetingKey: "your_targeting_key", structure: MutableStructure())
+OpenFeatureAPI.shared.setProvider(provider: provider, initialContext: context)
+``` 
+
+## Contributing
+
+Pull requests are welcome. For major changes, please open an issue first
+to discuss what you would like to change.
+
+Please make sure to update tests as appropriate.
+
+## License
+
+[Apache License 2.0](https://choosealicense.com/licenses/apache-2.0/)