Add Record Layer documentation

Add some design documentation on how the record layer works to aid future
maintenance.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

ssl/record/README

+Record Layer Design
+===================
+
+This file provides some guidance on the thinking behind the design of the
+record layer code to aid future maintenance.
+
+The record layer is divided into a number of components. At the time of writing
+there are four: SSL3_RECORD, SSL3_BUFFER, DLTS1_BITMAP and RECORD_LAYER. Each
+of these components is defined by:
+1) A struct definition of the same name as the component
+2) A set of source files that define the functions for that component
+3) A set of accessor macros
+
+All struct definitions are in record.h. The functions and macros are either
+defined in record.h or record_locl.h dependent on whether they are intended to
+be private to the record layer, or whether they form part of the API to the rest
+of libssl.
+
+The source files map to components as follows:
+
+dtls1_bitmap.c                                   -> DTLS1_BITMAP component
+ssl3_buffer.c                                    -> SSL3_BUFFER component
+ssl3_record.c                                    -> SSL3_RECORD component
+rec_layer_s23.c, rec_layer_s3.c, rec_layer_d1.c  -> RECORD_LAYER component
+
+The RECORD_LAYER component is a facade pattern, i.e. it provides a simplified
+interface to the record layer for the rest of libssl. The other 3 components are
+entirely private to the record layer and therefore should never be accessed
+directly by libssl.
+
+Any component can directly access its own members - they are private to that
+component, e.g. ssl3_buffer.c can access members of the SSL3_BUFFER struct
+without using a macro. No component can directly access the members of another
+component, e.g. ssl3_buffer cannot reach inside the RECORD_LAYER component to
+directly access its members. Instead components use accessor macros, so if code
+in ssl3_buffer.c wants to access the members of the RECORD_LAYER it uses the
+RECORD_LAYER_* macros.
+
+Conceptually it looks like this:
+
+                        libssl
+                           |
+---------------------------|-----record.h--------------------------------------
+                           |
+                    _______V______________
+                   |                      |
+                   |    RECORD_LAYER      |
+                   |                      |
+                   |    rec_layer_s23.c   |
+                   |          ^           |
+                   |          |           |
+                   |    rec_layer_s3.c    |
+                   |          ^           |
+                   | _________|__________ |
+                   ||                    ||
+                   || DTLS1_RECORD_LAYER ||
+                   ||                    ||
+                   || rec_layer_d1.c     ||
+                   ||____________________||
+                   |______________________|
+        record_locl.h     ^   ^   ^
+         _________________|   |   |_________________
+        |                     |                     |
+   _____V_________      ______V________      _______V________
+  |               |    |               |    |                |
+  | SSL3_BUFFER   |    | SSL3_RECORD   |    | DTLS1_BITMAP   |
+  |               |--->|               |    |                |
+  | ssl3_buffer.c |    | ssl3_record.c |    | dtls1_bitmap.c |
+  |_______________|    |_______________|    |________________|
+
+
+The three RECORD_LAYER source files build progressively on each other, i.e.
+the simplest is rec_layer_s23.c. This provides the most basic functions used
+for version negotiation. Next rec_layer_s3.c adds the SSL/TLS layer. Finally
+rec_layer_d1.c builds off of the SSL/TLS code to provide DTLS specific
+capabilities. It uses some DTLS specific RECORD_LAYER component members which
+should only be accessed from rec_layer_d1.c. These are held in the
+DTLS1_RECORD_LAYER struct.