document compile-time argument checking

Author: nerdralph

README.md

@@ -3,10 +3,11 @@ Small Arduino/Wiring core for 8-bit tiny AVRs: ATtiny13 and ATtiny25-85 series.
 
 Work in progress as of March 2020
 
-Goals: More than 50% of Arduino sketches run unmodified with picoCore, and 80% run with minor tweaks.
+Goals: More than 50% of Arduino sketches run unmodified with picoCore, and 80% run with minor tweaks.  Unlike the official AVR core, picoCore has compile-time checking of many arguments.  Calling digitalWrite(42) will cause a compile error with the message, "pin out of range". 
 
-Only those parts of the Arduino/Wiring API which can be implmented efficiently will be supported:
+Only those parts of the Arduino/Wiring API which can be implmented efficiently will be supported.
 
+# Development Status
 digitalRead, digitalWrite, pinMode - implemented in v0.1
 
 delay, delayMicroseconds - implemented in v0.1 using avr-gcc builtin delay functions.