Update README

Author: tylertreat

README.md

@@ -1,3 +1,65 @@
-# liftbridge-grpc
+# Liftbridge API
 
-Protobuf definitions for the [Liftbridge](https://github.com/liftbridge-io/liftbridge) gRPC API.
+This repository contains the public API definitions for
+[Liftbridge](https://github.com/liftbridge-io/liftbridge). It is primarily
+intended for Liftbridge client developers.
+
+## gRPC API
+
+The client-facing gRPC API is defined in [api.proto](api.proto).
+
+## Direct NATS API
+
+It is also possible for a client to publish messages to Liftbridge via NATS
+directly. Liftbridge accepts plain NATS messages, allowing it to make existing
+subjects durable without any publisher changes. However, these messages will
+not have some features such as acks.
+
+In order to opt into Liftbridge-specific features, the message must be prefixed
+with the following header and be encoded as a `Message` (defined in
+[api.proto](api.proto)).
+
+### Liftbridge Envelope Header
+
+```
+0               8               16              24              32
+├───────────────┴───────────────┴───────────────┴───────────────┤
+│                          Magic Number                         │
+├───────────────┬───────────────┬───────────────┬───────────────┤
+│    Version    │   HeaderLen   │     Flags     │    Reserved   │
+├───────────────┴───────────────┴───────────────┴───────────────┤
+│                       CRC-32C (optional)                      │
+└───────────────────────────────────────────────────────────────┘
+```
+
+#### Magic number [4 bytes]
+
+The Liftbridge magic number is `B9 0E 43 B4`. This was chosen by random but
+deliberately restricted to invalid UTF-8 to reduce the chance of a collision.
+This was also verified to not match known file signatures.
+
+#### Version [1 byte]
+
+The version byte allows for future protocol upgrades. This should only be
+bumped if the envelope format changes or if the message encoding changes in a
+non-backwards-compatible way. Adding fields to the messages should not require
+a version bump.
+
+#### HeaderLen [1 byte]
+
+The header length is the offset of the payload. This is included primarily for
+safety.
+
+#### Flags [1 byte]
+
+The flag bits are defined as follows:
+
+| Bit | Description     |
+| --- | --------------- |
+| 0   | CRC-32C enabled |
+
+#### CRC-32C [4 bytes, optional]
+
+The CRC-32C (Castagnoli) is the checksum of the payload (i.e. from HeaderLen to
+the end). This is optional but should significantly reduce the chance that a
+random NATS message is interpreted as a Liftbridge message.