Update README.md for Docker Official Image.

[Liam: rewrote commit message]
Acked-by: Liam Crilly <liam@nginx.com>
Cc: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

README.md

@@ -38,7 +38,7 @@ For details and available language packages, see the
 ### Docker
 
 ``` console
-$ docker pull docker.io/nginx/unit
+$ docker pull unit
 ```
 
 For a description of image tags, see the
@@ -71,6 +71,11 @@ For details and available language packages, see the
 
 ## Running a Hello World App
 
+Unit runs apps in a
+[variety of languages](https://unit.nginx.org/howto/samples/).
+Let's consider a basic example,
+choosing PHP for no particular reason.
+
 Suppose you saved a PHP script as `/www/helloworld/index.php`:
 ``` php
 <?php echo "Hello, PHP on Unit!"; ?>