Create CONTRIBUTING.md

Author: mrousavy

CONTRIBUTING.md

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+## Contributing to react-native-blurhash
+
+👍🎉 First off, thanks for taking the time to contribute! 🎉👍
+
+I initially created the library because I wanted to create a rich experience for the startup app I'm building, [Springsale](https://github.com/Springsale). 
+As **react-native-blurhash** gained more and more stargazers, I added more and more features to this library such as [encoding support](https://github.com/mrousavy/react-native-blurhash#encoding), asynchronous decoding, load status callbacks, [windows support](https://github.com/mrousavy/react-native-blurhash/issues/41) and more.
+
+### What you need to know
+
+1. The native iOS part is written in Swift and bridged to Objective-C (for the React-Native macros). If you're not familiar with this setup, either read into the code, or check out the [resources](https://github.com/mrousavy/react-native-blurhash#resources) I used.
+    1. The blurhash **encoder** and **decoder** is copied from [woltapp/blurhash/Swift](https://github.com/woltapp/blurhash/tree/master/Swift) as per commit [95d05e2](https://github.com/woltapp/blurhash/commit/95d05e2f639db8086c1b8bc53092a4cf5c14924b) and uses [this PR](https://github.com/woltapp/blurhash/pull/80) to cache Cosine operations
+2. The native Android part is written in Kotlin. If you're not familiar with this setup, read some Kotlin syntax and you'll get the hang of it.
+    1. The blurhash **decoder** is copied from [woltapp/blurhash/Kotlin](https://github.com/woltapp/blurhash/tree/master/Kotlin/lib/src/main/java/com/wolt/blurhashkt) as per commit [539cf8a](https://github.com/woltapp/blurhash/commit/539cf8a5425d766227b52e1d487a231bc5c7b72e)
+    2. The blurhash **encoder** is copied from [this PR](https://github.com/woltapp/blurhash/pull/55)
+3. The JS part is written in JS, and types are manually created using an `index.d.ts` file.
+4. Currently the `BlurhashView` type inherits from `UIView` (iOS) and `AppCompatImageView` (Android). That means, they are different types, and none of those inherits from `RCTImageView`/`ReactImage`. (Mostly because on Android the `setDrawable(...)` fucntion is deprecated when inheriting from `ReactImage` since it uses `source` for setting the Image source)
+
+
+### Code Style
+
+1. For Swift, use [SwiftFormat](https://github.com/nicklockwood/SwiftFormat)
+2. For Kotlin, use Android Studio
+3. For JS, use ESLint and Prettier (already configured in the Project)
+
+### Pull Requests
+
+Create pull requests if:
+
+* You added some stuff that enhances the library
+* You added some stuff that fixes a bug in the library
+* You changed some stuff so the library is more efficient (is _faster_)
+
+### Issues
+
+Create issues if:
+
+* Something doesn't work as expected
+* You have a question (use the _Question_ issue template)
+* You want to propose changes or enhancements to the library
+
+
+### Extra
+
+Always stay civil in discussions. Otherwise I'll find you