Fix: Changed --save to --save-dev in readme

[amilajack]

this probably fits most use-cases

[eslintbot]

Thanks for the pull request, @amilajack! I took a look to make sure it's ready for merging and found some changes are needed:

• The commit summary needs to begin with a tag (such as Fix: or Update:). Please check out our guide for how to properly format your commit summary and update it on this pull request.

• Pull requests with code require an issue to be mentioned at the end of the commit summary, such as (fixes #1234). Please update the commit summary with an issue (file a new issue if one doesn't already exist).

Can you please update the pull request to address these?

(More information can be found in our pull request guide.)

[JamesHenry]

LGTM

[JamesHenry]

Please note for further contributions that your commit message should have been Docs: Changed --save to --save-dev in readme. I amended this when merging.

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