Add msys2 and Cygwin instructions, also add .exe extension to built tools.

Todo: actually extensively test out msys2 and Cygwin, and double check the readme.

tools/rsfont/Makefile

@@ -8,10 +8,16 @@ SRCS = main.c convert_png.c util.c font.c
 
 .PHONY: all clean
 
-all: rsfont
+ifeq ($(OS),Windows_NT)
+EXE := .exe
+else
+EXE :=
+endif
+
+all: rsfont$(EXE)
 	@:
 
-rsfont: $(SRCS) convert_png.h gfx.h global.h util.h font.h
+rsfont$(EXE): $(SRCS) convert_png.h gfx.h global.h util.h font.h
 	$(CC) $(CFLAGS) $(SRCS) -o $@ $(LDFLAGS) $(LIBS)
 
 clean: