spell out we're doing 3PID invites

src/components/views/rooms/MemberList.js

@@ -254,7 +254,7 @@ module.exports = React.createClass({
         } else {
             return (
                 <form onSubmit={this.onPopulateInvite}>
-                    <input className="mx_MemberList_invite" ref="invite" placeholder="Invite another user"/>
+                    <input className="mx_MemberList_invite" ref="invite" placeholder="Invite user (email)"/>
                 </form>
             );
         }