Add photo to readme

Author: colemonnahan

README.md

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 Course materials for a TMB workshop at the Universidad de Concepción,
 January 2018.
 
+![group photo](https://github.com/colemonnahan/tmb_workshop/blob/master/group_photo.jpg "Participants")
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 Some material was modified with permission from a course by Jim Thorson at
 https://github.com/James-Thorson/2016_Spatio-temporal_models.