catalogue existing consumers

[keighrim]

Similar to #19, we'd like to catalogue existing consumer applications.

[keighrim]

https://github.com/clamsproject/mmif-visualizer
https://github.com/clamsproject/universal-viewer-visualizer
https://github.com/clamsproject/mmif-summarizer

So far we have these three.

[keighrim]

related:

• release model  mmif-visualizer#25
• Make summarizer available on PyPI mmif-summarizer#12