Added help text for incoming webhooks.

web/react/components/user_settings/manage_incoming_hooks.jsx

@@ -148,6 +148,8 @@ export default class ManageIncomingHooks extends React.Component {
 
         return (
             <div key='addIncomingHook'>
+                {'Create webhook URLs for channels and private groups. These URLs can be used by outside applications to create posts in any channels or private groups you have access to. The specified channel will be used as the default.'}
+                <br/>
                 <label className='control-label'>{'Add a new incoming webhook'}</label>
                 <div className='padding-top'>
                     <select