Create README

README.md

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+# AndroidDoubleRatchet
+
+Implementation of the <a href="https://www.signal.org/docs/specifications/doubleratchet/#external-functions">Double Ratchet</a> protocol in Kotlin for Android. The cryptographic operations are provided by <a href="https://github.com/terl/lazysodium-android.git">Lazysodium</a> entirely.
+
+## Installation
+
+### Jitpack
+To integrate the library via jitpack add the jitpack repository to your root `build.gradle` file:
+
+```
+allprojects {
+  repositories {
+    ...
+    maven { url  "https://dl.bintray.com/terl/lazysodium-maven" }
+    maven { url 'https://jitpack.io' }
+  }
+}
+```
+
+You can then add the dependency to your app's `build.gradle` file where `$VERSION` specifies the specific version of the library:
+
+```
+dependencies {
+  implementation 'com.github.TICESoftware:AndroidDoubleRatchet:$VERSION'
+  implementation "com.goterl.lazycode:lazysodium-android:4.1.0@aar"
+  implementation 'net.java.dev.jna:jna:5.5.0@aar'
+}
+ ```
+
+
+## Usage
+
+Alice and Bob calculate a shared secret using a secure channel. After that one party can start the conversation as soon as she gets to know the public key of the other one.
+
+```kotlin
+import com.ticeapp.androiddoubleratchet.DoubleRatchet
+
+val sharedSecret: ByteArray = ...
+val info = "DoubleRatchetExample"
+
+val bob = DoubleRatchet(keyPair = null, remotePublicKey = null, sharedSecret = sharedSecret, maxSkip = 20, maxCache = 20, info = info)
+
+// Bob sends his public key to Alice using another channel
+// sendToAlice(bob.publicKey)
+
+val alice = DoubleRatchet(keyPair = null, remotePublicKey = bob.publicKey, sharedSecret = sharedSecret, maxSkip = 20, maxCache = 20, info = info)
+
+// Now the conversation begins
+val message = "Hello, Bob!".encodeToByteArray()
+val encryptedMessage = alice.encrypt(message)
+val decryptedMessage = bob.decrypt(encryptedMessage)
+
+println(decryptedMessage.decodeToString()) // Hello, Bob!
+```