Explain extensionless behavior in getting started

<a href="https://github.com/daveaglick"><img src="https://avatars.githubusercontent.com/u/1020407?v=4" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [daveaglick](https://github.com/daveaglick)**
_Monday Aug 21, 2017 at 10:49 GMT_
_Originally opened as https://github.com/Wyamio/Wyam.Web/issues/51_

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