Update readme to include jspm step

[hhsadiq]

Hey @alexweber, first of all, thanks for such a wonderful boilerplate code for es6.

Can you add this step in Usage section of readme.

1. Install jspm globally by running npm install jspm -g

Because without this step the jspm install does not run and lib folder does not generate in src directory, although npm start was running fine without any error. I just have to spend some time to figure this out.

[alexweber]

Hello @hhsadiq thank you! :)

I'm sorry you had issues getting it to work, would you mind submitting a PR for the changes you described please?

Thanks!