Adding yarn option

[ghost]

Some developers may wish to use Yarn instead of npm, so I have added yarn alternative scripts.

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[ghost]

I signed it @googlebot

[googlebot]

CLAs look good, thanks!

[ghost]

Thanks @leo6104 have fixed the typo.

[jelbourn]

I think this needs something that clearly conveys it would be one or the other, not both.

[ghost]

@jelbourn something like

We have added install scripts so you can pick between npm or yarn

[jelbourn]

Right under "Step 1", it should say

"You can use either the npm or yarn command-line tool to install packages. Use whichever is appropriate for your project in the examples below."

[ghost]

@jelbourn added it 👍

[jelbourn]

LGTM

[ghost]

@jelbourn seems Travis didn't like it possibly because of the file being updated

[jelbourn]

The travis failure is unrelated (seems like some kind of flake)

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