suggest gulp-cli in the getting started

[phated]

Closes #1451

@contra How does this look?

[yocontra]

@phated is the collision an issue?

[phated]

In old versions of npm (ones that ship with many versions of node), if you did:

1. npm install -g gulp
2. npm install -g gulp-cli
3. npm rm -g gulp
   It would remove the gulp bin because it knew that gulp could create it but it didn't check if the bin was owned by gulp. We had this fixed as part of our issues in a 3.x release but I don't know if it ever got backported to 2.x and many people, myself included, still use 2.x for speed, etc.

[yocontra]

@phated Mind testing it on 2.x?

[phated]

I'd prefer not to, I have gulp-cli linked globally to work on it and the switching between linked and installed is a nightmare due to manpage generation. Even if this was fixed in 2.x, 1.4.x is still out in the wild and shipping with versions of node (0.10.x)

[yocontra]

@phated okay.