Update 3.1.0.md

I was trying 
Member:
  extensions:
    MyMemberExtension

And it didn't work then someone on IRC pointed that I need to put a '-' before values. So this works.
Member:
  extensions:
    - MyMemberExtension
Hope will help someone else. Attached screenshot of warning I was getting without '-'
![selection_019](https://f.cloud.github.com/assets/1092931/661209/7d12afca-d72b-11e2-904b-06848a146bbd.png)
.