docs: add readme

Signed-off-by: Erik Michelson <opensource@erik.michelson.eu>

README.md

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+<!--
+SPDX-FileCopyrightText: 2024 Erik Michelson <opensource@erik.michelson.eu>
+
+SPDX-License-Identifier: MIT
+-->
+
+# array-changeset
+
+This tiny library can be used to calculate the changeset between two arrays and applying these changes to another array.
+
+The library is **ESM-only**. See [this note by sindresorhus][esm-note] to learn more about that.
+
+## Installation
+
+### Using npm
+
+```shell
+npm install array-changeset
+```
+
+## Usage
+
+```javascript
+import { Changeset } from 'array-changeset'
+
+const arrayA = ['foo', '!']
+const arrayB = ['foo', 'bar']
+
+// Use fromDiff to create a changeset from the difference between two arrays
+const changeset = Changeset.fromDiff(arrayA, arrayB)
+
+// Check if there are any changes from arrayA to arrayB
+console.log(changeset.hasChanges()) // true
+
+// Access the added and removed elements
+console.log(changeset.getAdded()) // ['bar']
+console.log(changeset.getRemoved()) // ['!']
+
+// Modify the changeset (arrayA and arrayB are not modified)
+changeset.add('!')
+changeset.remove('bar')
+changeset.remove('more')
+changeset.uniquelyAddItem('unique', (a, b) => a === b) // The comparator is optional and defaults to fast-deep-equal
+changeset.uniquelyAddItem('unique', (a, b) => a === b)
+
+console.log(changeset.getAdded()) // ['unique']
+console.log(changeset.getRemoved()) // ['more']
+
+// Apply the changeset to another array (without modifying arrayC)
+const arrayC = ['much', 'more', 'foo', '!']
+const modifiedC = changeset.applyOnto(arrayC)
+console.log(modifiedC) // ['much', 'foo', '!', 'unique']
+
+// Clear the changeset
+changeset.clear()
+
+console.log(changeset.hasChanges()) // false
+```
+
+## License
+
+This project is licensed under the MIT License. See the [LICENSE](LICENSE) file for details.
+
+[esm-note]: https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c