Info on generating phpdocs

Author: jeffswartz

.gitignore

@@ -9,4 +9,5 @@ sample/*/cache/
 composer.lock
 
 docs/
-.phpdoc/
+.phpdoc/
+phpDocumentor*

DEVELOPING.md

@@ -22,7 +22,10 @@ This project's tests are written as PHPUnit test cases. Common tasks:
 
 ### Building docs
 
-Install [PhpDocumentor](https://docs.phpdoc.org/). Then run `php phpDocumentor.phar --config phpdoc.dist.xml`.
+Install [PhpDocumentor](https://docs.phpdoc.org/). Add it to the project root directory.
+Then run `php phpDocumentor.phar --config phpdoc.dist.xml`.
+
+The current reference docs were built with PhpDocumentor 3.3.0.
 
 ### Releasing