Some improvements and clarifications to the instructions/examples.

I tried to clean up some of the flow of the basic usage and I tried to add some clarifications to the confusing process of adding custom installers.  The two key points there that were previously omitted were that 1. the custom installer does not operate on the root package, and 2. the composer-plugin-api must be required.

In basic usage, I removed the paragraph that attempted to explain why vendor names and package names are often the same.  Sorry, but I have no idea what you're talking about and I've been reading that blurb and using composer for over a year.  E.g. this bit: "For projects with a unique name" -- what does that mean?  Doesn't *every* project have a unique name?   If you want to elaborate on the use case, then please come up with a valid explanation and example.  For now I've removed that because I think it helps your readers from sinking into confusion.