Explain how to use watchr in the README

Support has been added for watchr, and this is very helpful while
walking the path to enlightenment as it keeps the users focus on
the koans and not on the repeated need to run rake after each edit.

However, only an experienced Rubyist would know how to install and
use watchr (or worse, they might just not notice the watchr config
file). So let's add an optional section to the README explaining
how to use watchr.
Explain how to use Watchr in the README

The koans have support for Watchr now, and this is very helpful
as you can focus on editing and not on re-running rake over and
over. However, a user new to Ruby will not even know what Watchr
is (I'd not used it before! I was using Guard) so they might miss
this choice.

There are some other minor edits for style and consistency. I explain
that Koans are tests. We/You tidy up. Removed some excess wordage.
Made some "your should do this" statements more imperative. Made
mark up more consistent.
Merge pull request #2 from TheTeaNerd/dk-explain-watchr-in-README

Explain Watchr use in README