Add basic README.

libaxolotl/README.md

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+# Overview
+
+The axolotl protocol is a "ratcheting" forward secrecy protocol that works in synchronous and
+asynchronous messaging environments.  The protocol overview is available [here](https://github.com/trevp/axolotl/wiki),
+and the details of the wire format are available [here](https://github.com/WhisperSystems/TextSecure/wiki/ProtocolV2).
+
+## PreKeys
+
+This protocol implementation heavily leverages the concept of PreKeys.  A PreKey is simply an
+ECPublicKey and unique PreKey ID.  At install time, clients generate a large list of PreKeys
+and transmit them to the server.  The server will then remove PreKeys from that list and hand
+them to other clients when requested.  A single PreKey can never be used twice.
+
+## Sessions
+
+The axolotl protocol is session-oriented.  Clients establish a "session," which is then used for
+all subsequent encrypt/decrypt operations.  There is no need to ever tear down a session once one
+has been established.
+
+Sessions are established in one of three ways:
+
+1. PreKeys. A client that wishes to send a message to a recipient can establish a session by
+   retrieving a PreKey for that recipient from the server.
+1. PreKeyWhisperMessages.  A client can receive a PreKeyWhisperMessage from a recipient and use it
+   to establish a session.
+1. KeyExchangeMessages.  Two clients can exchange KeyExchange messages to establish a session.
+
+## State
+
+An established session encapsulates a lot of state between two clients.  That state is maintained
+in durable records which need to be kept for the life of the session.
+
+State is kept in the following places:
+
+1. Identity State.  Clients will need to maintain the state of their own identity key pair, as well
+   as identity keys received from other clients.
+1. PreKey State. Clients will need to maintain the state of their generated PreKeys.
+1. Session State.  Clients will need to maintain the state of the sessions they have established.
+
+# Using libaxolotl
+
+## Install time
+
+At install time, a libaxolotl client needs to generate its identity keys, registration id, and
+prekeys.
+
+```
+IdentityKeyPair    identityKeyPair = KeyHelper.generateIdentityKeyPair();
+int                registrationId  = KeyHelper.generateRegistrationId();
+List<PreKeyRecord> preKeys         = KeyHelper.generatePreKeys(startId, 100);
+
+// Store identityKeyPair somewhere durable and safe.
+// Store registrationId somewhere durable and safe.
+// Store preKeys somewhere durable and safe.
+
+```
+
+## Building a session
+
+A libaxolotl client needs to implement three interfaces: IdentityKeyStore, PreKeyStore, and
+SessionStore.  These will manage loading and storing of identity, prekeys, and session state.
+
+Once those are implemented, building a session is fairly straightforward:
+
+```
+SessionStore     sessionStore  = new MySessionStore();
+PreKeyStore      preKeyStore   = new MyPreKeyStore();
+IdentityKeyStore identityStore = new MyIdentityKeyStore();
+
+// Instantiate a SessionBuilder for a remote recipientId + deviceId tuple.
+SessionBuilder sessionBuilder = new SessionBuilder(sessionStore, preKeyStore, identityStore,
+                                                   recipientId, deviceId);
+
+// Build a session with a PreKey retrieved from the server.
+sessionBuilder.process(retrievedPreKey);
+
+SessionCipher sessionCipher = new SessionCipher(sessionStore, recipientId, deviceId);
+CiphertextMessage message = sessionCipher.encrypt("Hello world!".getBytes("UTF-8"));
+
+deliver(message.serialize());
+```