Add an entry point for the V3 app (#450)

Summary:
This implements Step 3 of the plan described in #448.

Test Plan:
Run `yarn start` and navigate to `/v3` (by clicking the nav link).

wchargin-branch: branch-v3

src/bridge/app/index.js

@@ -9,6 +9,7 @@ import {
 } from "react-router-dom";
 import "./index.css";
 import V1App from "../../v1/app/App";
+import V3App from "../../v3/app/App";
 import registerServiceWorker from "./registerServiceWorker";
 
 const root = document.getElementById("root");
@@ -23,10 +24,14 @@ ReactDOM.render(
         <li>
           <NavLink to="/v1">V1</NavLink>
         </li>
+        <li>
+          <NavLink to="/v3">V3</NavLink>
+        </li>
       </ul>
       <Route exact path="/" render={() => <Redirect to="/v1" />} />
       <hr />
       <Route path="/v1" component={V1App} />
+      <Route path="/v3" component={V3App} />
     </React.Fragment>
   </Router>,
   root

src/v3/app/App.js

@@ -0,0 +1,30 @@
+// @flow
+
+import React from "react";
+import {Route, NavLink, type Match} from "react-router-dom";
+
+export default class App extends React.Component<{match: Match}> {
+  render() {
+    const {match} = this.props;
+    return (
+      <div>
+        <nav>
+          <ul>
+            <li>
+              <NavLink to={match.url}>Home</NavLink>
+            </li>
+          </ul>
+        </nav>
+
+        <hr />
+        <Route exact path={match.url} component={Home} />
+      </div>
+    );
+  }
+}
+
+const Home = () => (
+  <div>
+    <h1>Welcome to SourceCred! Please make yourself at home.</h1>
+  </div>
+);