add info about running the demo

[tgpetrov]

• I added information with steps to run the demo, while now they seem obvious to me, the first time I tried using CodePush there were not so easy to guess.
• adding an app seems to currently require specifying nativescript so I added it to the docs as well, not sure if the documentation is wrong or it is a bug that the documented script does not work
• minor change in a column alignment

[EddyVerbruggen]

Hey Todor, thanks a million for these changes. Very helpful!

I have every intention to remove the need to specify nativescript, so I'm happy you flagged these issues and I'll look at the CLI to figure out where I forgot to change the implementation.

[EddyVerbruggen]

I've just released nativescript-code-push-cli version 1.0.6 which fixes the mandatory nativescript param. That'll no longer be needed.