add Vagrantfile to support cross-platform development

Update CONTRIBUTING to reflect plugin split

Vagrantfile

@@ -0,0 +1,21 @@
+# -*- mode: ruby -*-
+# vi: set ft=ruby :
+
+Vagrant.configure(2) do |config|
+  config.vm.box = "bento/ubuntu-16.04"
+
+  config.vm.synced_folder "..", "/go/src/github.com/containernetworking"
+
+  config.vm.provision "shell", inline: <<-SHELL
+    set -e -x -u
+
+    apt-get update -y || (sleep 40 && apt-get update -y)
+    apt-get install -y git
+
+    wget -qO- https://storage.googleapis.com/golang/go1.8.3.linux-amd64.tar.gz | tar -C /usr/local -xz
+
+    echo 'export GOPATH=/go' >> /root/.bashrc
+    echo 'export PATH=$PATH:/usr/local/go/bin:$GOPATH/bin' >> /root/.bashrc
+    cd /go/src/github.com/containernetworking/plugins
+  SHELL
+end