get Cropper instance documentation

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PR Checklist

Please check if your PR fulfills the following requirements:

• [ ] The commit message follows our guidelines.
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update
[ ] Refactor
[ ] Build related changes
[x ] Documentation content changes
[ ] Other, Please describe:

What is the current behavior?

Documentation contains information about manipulation with Cropper instance, but if you use jquery $('img').cropper returns DOM object and save instance to data attribute

Issue Number: N/A

What is the new behavior?

Added to documentation how to get Cropper instance

Does this PR introduce a breaking change?

[ ] Yes
[ x ] No

Other information