Adding FRETENATOR plugin wiki page

[JimageJ]

Adds a wiki page for FRETENATOR segmentation, ratiometric analysis and ROI labelling plugins.

The page covers installation via update site (currently waiting for the pull request to the list of update sites), output explanation, plugin usage, links to tutorial video tutorials.

[ctrueden]

Thanks @JimageJ for the documentation, and thanks @tferr for merging.

@JimageJ I invited you to join this repository as a collaborator, so that you can edit directly in the future without needing to make PRs. (Although if you prefer making PRs, that is still OK too—up to you.)

[JimageJ]

Thanks for the merge @tferr!
Thanks for inviting me to be a collaborator @ctrueden! It should make edits easier in the future.