Some documentation improvements (RMagick migration, examples)

I've added a few things here (see commits). Mostly my aim is to make starting easier for people who are used to RMagick and help them know how to know what command to use.

Not included in this, is an idea I've got. Instead of having all these examples in the main README.md we should move it out to an examples directory with one file/example. That way we can provide examples of pretty much everything neatly. Thoughts?