Fix mailing list link

The domain should be scif.io.
Fix SciJava spelling
Use em dash where appropriate
Mention that Bio-Formats is an OME product
Add link to SCIFIO-BF-compat project
Use simpler URL for Bio-Formats link
Clean up formatting

* Wrap text at 80 chars
* Put <p> elements on own line
Describe update sites as an ImageJ2 feature

They were incubated in Fiji, but are now available for all users of
ImageJ2, not just Fiji installations.