Update README.md
Merge branch 'numeric'
update README
fix build error on OCaml <= 4.00
add test files: {error,}-unknown1.sfp
allow unknown value
add unknown-{goal,init}.sfp
fix bugs for True/False constraints
allow unknown value in initial state
rename unknown-{init,goal}.sfp -> unknown1-{init,goal}; add test files unknown2-{init,goal}.sfp