Add protocol overview

mirai-core-qqandroid/src/commonMain/kotlin/net/mamoe/mirai/qqandroid/network/protocol/README.md

+# QQAndroid Protocol
+
+## Overview
+
+Note: `head` and `body` functions does nothing. They just work as
+notations
+
+PseudoCode:
+```
+OutgoingPacket { 
+    head {
+        int      remaining.length + 4
+        int      0x0A
+        byte     0x02
+        int      extra data size + 4
+        byte[]   extra data // initially={}
+        byte     0
+        int      uinAccount.length + 4
+        byte[]   uinAccount // =qqNumber.toString()
+    } 
+    body { // encrypted by `ByteArray(16)` when login, after which by sessionKey
+        SSOPacket {
+            int head.size + 4
+            head {
+                int      sequenceId
+                int      subAppId
+                int      subAppId
+                hex      "01 00 00 00 00 00 00 00 00 00 01 00" // unknown values
+                int      extraData.size + 4
+                byte[]   extraData
+                int      commandName.length + 4
+                byte[]   commandName
+                int      4 + 4
+                int      0x02B05B8B
+                int      imei.length + 4
+                byte[]   imei
+                int      0 + 4
+                int      ksid.length + 4
+                byte[]   ksid
+                int      0 + 4
+            }
+            
+            int body.size + 4
+            body {
+                OicqRequestPacket {
+                    head {
+                        byte     2 // head flag
+                        short    27 + 2 + remaining.length
+                        ushort   client.protocolVersion // const 8001
+                        ushort   0x0001
+                        uint     client.account.id
+                        byte     3 // const
+                        ubyte    encryptMethod.value // [EncryptMethod]
+                        byte     0 // const
+                        int      2 // const
+                        int      client.appClientVersion
+                        int      0 // const
+                    }
+                    
+                    body {
+                        // only write one of the following two structures!!
+                    
+                        // if encryption method is ECDH
+                        EncryptionMethodECDH {
+                            head {
+                                byte     1
+                                byte     1
+                                byte[]   [ECDH.privateKey]
+                                short    258
+                                short    [ECDH.publicKey].size // always 49
+                                byte[]   [ECDH.publicKey]
+                            }
+                            
+                            body {
+                                // real body
+                            }
+                        }
+                        
+                        // if encryption method is SessionKey
+                        EncryptionMethodSessionKey {
+                            head {
+                                byte     1
+                                byte     if (currentLoginState == 2) 3 else 2
+                                fully    key
+                                short    258
+                                short    0
+                            }
+                            
+                            body {
+                                // real body
+                            }
+                        }
+                    }
+                    tail {
+                        byte     3 // tail flag
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+## Packet bodies
+
+### LoginPacket - SubCommand 9
+**TO BE UPDATED**
+
+PseudoCode:
+```
+short 9 // subCommand
+tlvList {
+    
+}
+```