Installation instructions should include npm

[Soviut]

Only showing yarn add nuxt-env in the README had one of our junior developers thinking that the module needed to be installed with Yarn, while the rest of our code base uses npm.

I tried forking and creating a PR but can't seem to target your master branch. So here it is Soviut#1

[samtgarson]

It's pretty standard practice to give one example in a README—your junior developer would only make that mistake once.

Others have been able to open PRs, if you want to give that another try I'd be happy to merge it.

[Soviut]

I wouldn't consider the installation instructions an example. Most packages I've seen show both.

I'll see about retrying the PR.