doc: Added python SimpleHTTPServer as a method reading docs in browser

Documented an easier way for users to browse documentation without
setting up a whole web environment. It make the process of reading the
latest documentation, and comparing it with the documentation of
older versions on the Node.JS website a lot easier.

BUILDING.md

@@ -206,14 +206,15 @@ To read the documentation:
 $ man doc/node.1
 ```
 
-If you prefer to read the documentation in the browser,
-run the following once `make doc` is done:
+If you prefer to read the documentation in the browser and have `python` 
+installed, you can run:
 
 ```console
-$ make docopen
+$ cd out/doc/api/
+$ python -m SimpleHTTPServer
 ```
 
-This will open up a browser tab/window with the documentation.
+and navigate to `http://localhost:8000` in your browser.
 
 To test if Node.js was built correctly: