update examples documentation

[KrisThielemans]

The READMEs for the examples are out-of-date and incomplete. There's more on our wiki , see e.g. https://github.com/SyneRBI/SIRF/wiki/Examples, https://github.com/SyneRBI/SIRF/wiki/Examples-(MR), https://github.com/SyneRBI/SIRF/wiki/Examples-(PET) but even that needs work.

I think we should move text from the wiki to the READMEs in this repo, and let the wiki point to them. It's the only way to keep things updated and in syn, with the resources that we have.