Adds os-server-groups

This commit adds support for the os-server-groups extension. This allows
users to create scheduling policies for launching instances.
diff --git a/openstack/compute/v2/extensions/servergroups/doc.go b/openstack/compute/v2/extensions/servergroups/doc.go
new file mode 100644
index 0000000..1e5ed56
--- /dev/null
+++ b/openstack/compute/v2/extensions/servergroups/doc.go
@@ -0,0 +1,2 @@
+// Package servergroups provides the ability to manage server groups
+package servergroups