note linux dependency in local getting-started-guide

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 adds a requirements section and points
users in the direction of guides more likely to work for them.
Merge pull request #920 from josephholsten/patch-1

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