added 404 page

src/App.js

@@ -2,14 +2,16 @@ import { BrowserRouter as Router, Switch, Route /*Link*/ } from "react-router-do
 import { Home } from "./pages/Home";
 import { About } from "./pages/About";
 import { Users } from "./pages/Users";
+import { NotFound } from "./pages/NotFound";
 
 export const App = () => {
     return (
         <Router>
             <Switch>
-                <Route path="/about" component={About} />
-                <Route path="/users" component={Users} />
-                <Route path="/" component={Home} />
+                <Route exact path="/about" component={About} />
+                <Route exact path="/users" component={Users} />
+                <Route exact path="/" component={Home} />
+                <Route component={NotFound} />
             </Switch>
         </Router>
     );

src/pages/NotFound/index.js

@@ -0,0 +1,3 @@
+export const NotFound = () => {
+    return <h1>Podana strona nie została odnaleziona</h1>;
+};