add ability to use global zeroclipboard script in readme.md

Thanks for your work! Saved me the headache of working with zeroclipboard's not fun api. I just wanted to suggest adding the option that you can globally override the cdnjs script in the readme, which I found mentioned here -> (https://github.com/brigand/react-zeroclipboard/issues/4). I was trying to see if this was possible myself when I came upon the closed issue, so it could just help others from searching around. I'll be happy to provide a PR with the addition if you'd like. Thanks again.

Read readme.md and the referenced zeroclipboard discussion at issue 4 to understand the existing CDN script usage and the requested global override. Update the README to document how to use a globally overridden zeroclipboard script, with the relevant option or example clearly shown.

Written by the indexing model from the issue text.