Add warning in EuiPortal about inability to render immediately. (#593)

src/components/portal/portal.js

@@ -1,6 +1,12 @@
 /**
  * NOTE: We can't test this component because Enzyme doesn't support rendering
  * into portals.
+ *
+ * NOTE: You **cannot** immediately return a EuiPortal from within the render method! This is
+ * because the portalNode doesn't exist until **after** it's mounted. In its current form, EuiPortal
+ * can only be used by components which are hidden or otherwise not rendered initially, like
+ * dropdowns and modals. If we want to support components wrapped in EuiPortal being visible
+ * immediately we can update EuiPortal to accept a DOM node as a prop, which should solve the problem.
  */
 
 import { Component } from 'react';