Update doc server example to work from any directory (facebook#1988)

* Node.js serving with absolute path

It’s safer to use the absolute path of the directory that you want to serve, in case you run the express app from another directory.

* Update README.md

template/README.md

@@ -1242,10 +1242,10 @@ const express = require('express');
 const path = require('path');
 const app = express();
 
-app.use(express.static('./build'));
+app.use(express.static(path.join(__dirname, 'build')));
 
 app.get('/', function (req, res) {
-  res.sendFile(path.join(__dirname, './build', 'index.html'));
+  res.sendFile(path.join(__dirname, 'build', 'index.html'));
 });
 
 app.listen(9000);
@@ -1264,11 +1264,11 @@ If you use routers that use the HTML5 [`pushState` history API](https://develope
 This is because when there is a fresh page load for a `/todos/42`, the server looks for the file `build/todos/42` and does not find it. The server needs to be configured to respond to a request to `/todos/42` by serving `index.html`. For example, we can amend our Express example above to serve `index.html` for any unknown paths:
 
 ```diff
- app.use(express.static('./build'));
+ app.use(express.static(path.join(__dirname, 'build')));
 
 -app.get('/', function (req, res) {
 +app.get('/*', function (req, res) {
-   res.sendFile(path.join(__dirname, './build', 'index.html'));
+   res.sendFile(path.join(__dirname, 'build', 'index.html'));
  });
 ```