Add a state machine README

Add some documentation on the thinking behind the state machine.
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

ssl/statem/README

+State Machine Design
+====================
+
+This file provides some guidance on the thinking behind the design of the
+state machine code to aid future maintenance.
+
+The state machine code replaces an older state machine present in OpenSSL
+versions 1.0.2 and below. The new state machine has the following objectives:
+    - Remove duplication of state code between client and server
+    - Remove duplication of state code between TLS and DTLS
+    - Simplify transitions and bring the logic together in a single location
+      so that it is easier to validate
+    - Remove duplication of code between each of the message handling functions
+    - Receive a message first and then work out whether that is a valid
+      transition - not the other way around (the other way causes lots of issues
+      where we are expecting one type of message next but actually get something
+      else)
+    - Separate message flow state from handshake state (in order to better
+      understand each)
+      - message flow state = when to flush buffers; handling restarts in the
+        event of NBIO events; handling the common flow of steps for reading a
+        message and the common flow of steps for writing a message etc
+      - handshake state = what handshake message are we working on now
+    - Control complexity: only the state machine can change state: keep all
+      the state changes local to the state machine component
+
+The message flow state machine is divided into a reading sub-state machine and a
+writing sub-state machine. See the source comments in statem.c for a more
+detailed description of the various states and transitions possible.
+
+Conceptually the state machine component is designed as follows:
+
+                        libssl
+                           |
+---------------------------|-----statem.h--------------------------------------
+                           |
+                    _______V____________________
+                   |                            |
+                   |    statem.c                |
+                   |                            |
+                   |    Core state machine code |
+                   |____________________________|
+        statem_locl.h     ^          ^
+                 _________|          |_______
+                |                            |
+   _____________|____________   _____________|____________
+  |                          | |                          |
+  | statem_clnt.c            | | statem_srvr.c            |
+  |                          | |                          |
+  | TLS/DTLS client specific | | TLS/DTLS server specific |
+  | state machine code       | | state machine code       |
+  |__________________________| |__________________________|
+               |        |_______________|__       |
+               |        ________________|  |      |
+               |       |                   |      |
+   ____________V_______V________   ________V______V_______________
+  |                             | |                               |
+  | statem_both.c               | | statem_dtls.c                 |
+  |                             | |                               |
+  | Non core functions common   | | Non core functions common to  |
+  | to both servers and clients | | both DTLS servers and clients |
+  |_____________________________| |_______________________________|
+