Linux users must enable NFS for Vagrant

docs/Running-Mastodon/Vagrant-guide.md

@@ -17,6 +17,8 @@ To create and provision a new virtual machine for Mastodon development:
     cd mastodon
     vagrant up
 
+**Note:** On Linux hosts, you will need to [enable NFS support](https://www.vagrantup.com/docs/synced-folders/nfs.html).
+
 Running `vagrant up` for the first time will run provisioning, which will:
 
 - Download the Ubuntu 14.04 base image, if there isn't already a copy on your machine
@@ -61,4 +63,4 @@ To run the `rspec` tests and `rubocop` style checker, you may either:
 
 ## Support/help
 
-If you are confused, or having any issues with the above, the Mastodon IRC channel ( irc.freenode.net #mastodon ) is a good place to find assistance.
+If you are confused, or having any issues with the above, the Mastodon IRC channel ( irc.freenode.net #mastodon ) is a good place to find assistance.