Added readme.txt file with explanations about the AT91SAM7S port.

bacnet-stack/ports/at91sam7s/readme.txt

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+This port was done with a AT91SAM7S-EK which contained a
+AT91SAM7S64 processor.  The compiler was the GNU ARM compiler
+and tools from Yagarto project.  
+
+The hardware was modified by severing the I-PA5 (RXD0), 
+I-PA6 (TXD0), I-PA7 (RTS0) pads and rerouting those 
+signals to a DS75176 RS-485 transceiver. 
+PIN SIGNAL AT91SAM7S
+--- ------ ---------
+ 1   RO      RXD0
+ 2   /RE     RTS
+ 3   DE      RTS
+ 4   DI      TXD0
+ 5   GND     GND
+ 6   DO      n/c
+ 7   DO      n/c
+ 8   +5V     From EXT_VCC via 5V Regulator
+
+The makefile allows you to build just the dlmstp or a simple
+server, both for programming into the flash memory of the processor.
+The dlmstp is the datalink layer for MS/TP over RS-485.
+
+I used the makefile from the command line on Windows, and 
+then used the SAM-BA to send the resulting .bin file to the 
+board using a J-Link.
+
+I got the crt.s, at91sam7s256.ld, blinker.c, init.c, isr.c, and
+timer.c from James P Lynch.  I created the rs485.c based on the 
+initialization sequence from serial.c by Keil Electronik.  I 
+got the at91sam7s256.h file from Atmel via the Keil website.
+I started with the makefile from James P Lynch, but it didn't work
+for me.  I then used some ideas from FreeRTOS makefile, and 
+created my own makefile from scratch.
+
+Hopefully you find it useful!
+
+Steve Karg