Explain how to use library for simple point arithmetic

[tomato42]

Since one of the stated goals of the library is to be a teaching tool for ECC, we should have documentation showing how you can use it to perform some simple operations on the the elliptic points.

Use the simple y**2 = x**3 + 1x + 4 mod 5 curve in examples (with generator (1, 1))

see also #95