Mention -Y flag for yarn users

[enriquecastl]

When I ran sb-create in my project, I always got this error. After debugging, I noticed that sb-create did not recognize that my project uses yarn properly. Passing the -Y flag fixed the issue. I think it'd be beneficial to make mention of this flag.

 Error: spawnSync npm.cmd ENOENT
    at _errnoException (util.js:1024:11)
    at spawnSync (child_process.js:579:20)
    at exports.installDeps (/usr/local/lib/node_modules/sb-cli/lib/helpers.js:100:14)
    at Object.<anonymous> (/usr/local/lib/node_modules/sb-cli/bin/generate.js:66:5)
    at Module._compile (module.js:635:30)
    at Object.Module._extensions..js (module.js:646:10)
    at Module.load (module.js:554:32)
    at tryModuleLoad (module.js:497:12)
    at Function.Module._load (module.js:489:3)
    at Function.Module.runMain (module.js:676:10)
. ✖

     An error occured while installing dependencies`.

[freeman14]

@enriquecastl Thanks for updated