Add demo from README as an example

examples/readme-demo.nbt

@@ -0,0 +1,7 @@
+use units::currencies
+
+8 km / (1 h + 25 min)
+140 € -> GBP
+atan2(30 cm, 1 m) -> deg
+let ω = 2π c / 660 nm
+print("Energy of red photon: {ℏ ω -> eV}")