Installation instructions could be clearer

Hi, complete clojurescript newb here. After completing the steps in the `Install` section of the readme, there's an `Update` where it tells you that you can update `boot` by running `boot -u`. I think that I was supposed to do this after getting the wrapper on my machine, but that was not clear to me when I started, so I skipped that step and went straight to `boot -h` which did apparently work but not exactly as described in the readme (I had to execute `boot -h` twice before getting the help info to appear.)

The `Update section` does state:

> With the wrapper installed you can update Boot's JAR files and keep up to date with the following command:

But I read that as "you could do this, in the future, when you need to update", not "you should do this right now".