update README.md
add ! subshell cmd
add v sqrt cmd
handle files
add ^ exponentiation cmd
fix operand order
add negative numbers and stack control commands
scan decimal points, ignore blanks, add more ops

Full floating point numbers like 3.14 are scanable.
All blank characters like spaces, tabs and newlines are ignored.
Added multiplication '*', division '/', modulus '%', quit 'q'.
scan numbers
write initial interface, add Makefile
initial commit