Adding users group blog post

I've written a blog post about the users' group I've organized at Davis, and how it can be a complementary tool to SWC workshops.

I haven't rebuilt the site.  When I run `make cache` I'm getting the following error:

```
mentat : ~/Dropbox/code/site > make cache
cp ./config/workshops_saved.yml ./_workshop_cache.yml
Request for https://raw.github.com/gvwilson/2014-09-29-iDigBio/gh-pages/index.html returned status code 404
```

The URL returns 404 in my browser, as well.