doc: Added sample command to test iojs build

PR-URL: nodejs#850
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>

README.md

@@ -119,6 +119,12 @@ To read the documentation:
 $ man doc/iojs.1
 ```
 
+To test if io.js was built correctly:
+
+```
+$ iojs -e "console.log('Hello from io.js ' + process.version)"
+```
+
 ### Windows
 
 Prerequisites:
@@ -140,6 +146,12 @@ To run the tests:
 > vcbuild test
 ```
 
+To test if io.js was built correctly:
+
+```
+$ iojs -e "console.log('Hello from io.js ' + process.version)"
+```
+
 ### Android / Android based devices, aka. Firefox OS
 
 Be sure you have downloaded and extracted [Android NDK]