Add instruction to enlarge serial buffer on arduino

Documentation/Laser/README.md

+K40 laser rasterized images sends fast and large size commands over serial,
+so, the common 64 byte buffer in arduino isn't enough.
+
+You can edit "board.txt" in arduino ide, copy the "mega 2560" board declaration, call it 
+"mega 2560 serial buffer 256" and then add a line like:
+
+```
+mega256.build.extra_flags=-DSERIAL_RX_BUFFER_SIZE=256 -DSERIAL_TX_BUFFER_SIZE=256
+```
+
+and then in the arduino ide select "mega 2560 serial buffer 256" board.
+