examples: Add chat example that also doesn't work properly yet

examples/chat/protocol.md

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+# Chat Protocol
+
+This document describes a simple chat protocol. To re-generate the source files,
+run `go run ./cmd/hopp-generate examples/chat/protocol.md examples/chat/protocol`
+
+## Messages
+
+### 0000 Error
+| Tag | Name        | Type   | Required |
+| --: | ----------- | ------ | -------- |
+|   0 | Code        | U16    | Yes      |
+|   1 | Description | String | No       |
+
+Error is sent by a party when the other party has done something erroneous. The
+valid error codes are:
+
+- 0: General, unspecified error 
+
+The description field, if specified, determines a human-readable error to be
+shown to the user. The sending party must immediately close the transaction
+after this message is sent.
+
+### 0001 Success
+Success is sent by a party when it has successfully completed a task given to it
+by the other party. The sending party must immediately close the transaction
+after this message is sent.
+
+### 0100 UpdateProfile
+| Tag | Name     | Type   | Required |
+| --: | -------- | ------ | -------- |
+|   0 | Nickname | String | No       |
+
+UpdateProfile is sent by the client in a new transaction to update the profile
+details that will be shown to other connected clients.
+
+### 0200 Join
+| Tag | Name     | Type   | Required |
+| --: | -------- | ------ | -------- |
+|   0 | Room     | String | Yes      |
+
+Join is sent by the client when it wishes to join a room. It must begin a new
+transaction, and that transaction will persist while the user is in that room.
+Messages having to do with the room will be sent along this transaction. To
+leave the room, the client must close the transaction.
+
+### 0201 Chat
+| Tag | Name     | Type   | Required |
+| --: | -------- | ------ | -------- |
+|   0 | Nickname | String | No       |
+|   1 | Content  | String | Yes      |
+
+Chat is sent by the client when it wishes to post a message to the room. It is
+also relayed by the server to other clients to notify them of the message. It
+must be sent within a room transaction.
+
+### 0300 JoinNotify
+| Tag | Name     | Type   | Required |
+| --: | -------- | ------ | -------- |
+|   0 | Nickname | String | No       |
+
+JoinNotify is sent by the server when another client joins the room. It must be
+sent within a room transaction.
+
+### 0301 LeaveNotify
+| Tag | Name     | Type   | Required |
+| --: | -------- | ------ | -------- |
+|   0 | Nickname | String | No       |
+
+LeaveNotify is sent by the server when another client leaves the room. It must
+be sent within a room transaction.