Update warning about Node versions

[lauriii]

What:

Update README.md to give warning on node.js version requirements to users installing latest version of cross-env.

Why: The documentation is currently warning about the node.js version requirements of the previous major release.

How: Edited README.md

Checklist:

• [X] Documentation
• [-] Tests
• [X] Ready to be merged

[codecov[bot]]

Codecov Report

Merging #225 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #225   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines          78     78           
  Branches       18     18           
=====================================
  Hits           78     78

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[kentcdodds]

@all-contributors please add @lauriii for docs

[allcontributors[bot]]

@kentcdodds

I've put up a pull request to add @lauriii! :tada:

[kentcdodds]

:tada: This PR is included in version 7.0.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: