adding build instructions

BUILD.txt

@@ -7,6 +7,19 @@ First step, build some parts of the core library :
 	$ cd core
 	$ make t
 	$ ./t -v
+	$ cd ..
+
+Then you must have datas for working on. One source of datas
+is the four values who came from the Arduino over serial line.
+
+	$ cd serial
+	$ make t
+
+Have a look at the 'essai.sh' file for an example of simple
+usage of the serial input. For generating all that numbers,
+you have to run some code on the Arduino mega. Sources are
+in the simulator/ folder.
+