Improved usage example

Glaive still needed a usage example, but the one I added previously left
a lot to be desired.

The new example uses helloworld, which everyone has by default, instead
of getting them to install my plugin.  :-)  It also omits the lengthy
exposition on the distinction between rtp-adding and installing, since
after PR1 gets committed, Glaive users won't need to care about that.
Best of all, it's a fully working example.
Use maktaba#Maktaba() instead of maktaba#plugin
Merge pull request #5 from chiphogg/fix_readme

Improved usage example