README: Add a section that explains basic usage of gapbuffer

[shymega]

Hi there,

I'm lookin' at using gapbuffer for my text editor project, and I think it would be great to have a section in the README that explains the basic usage of gapbuffer, perhaps even with some examples in an examples/ directory..?

This would also be good for new users to the gapbuffer project as well.

Thanks for your time.

[WarpspeedSCP]

Yes, please!