docs/getting-started-guides/locally.md: note linux dependency

While the linux dependency is obvious with a moment's thought, some users (especially on *nix) might plow ahead before realizing their system isn't supported. This points them in the direction of guides more likely to work for them.
How about something like:

```
### Requirements 

 - Linux 
 - Docker 1.0.0+

Not running Linux? Consider running Linux in a local virtual machine with [Vagrant](vagrant.md), or on a cloud provider like [Google Compute Engine](gce.md)
```