Add instruction to enable phar extension in README

Author: weynhamz

README.md

@@ -25,9 +25,10 @@ should be easy though (Apologize for not being able to provide any help for
 this, I am really not a Windows guy), it also would be great if someone could
 provide a patch for this.
 
-Installation is simple, just run `make` in the root directory of the source,
-you will get a `phpctags` PHAR executable, add it to your `$PATH`, then you
-can invoke `phpcatgs` directly from anywhere.
+Installation is simple, make sure you have PHP's PHAR extension enabled, then
+just run `make` in the root directory of the source, you will get a `phpctags`
+PHAR executable, add it to your `$PATH`, then you can invoke `phpcatgs`
+directly from anywhere.
 
 See [phpctags on packagist](http://packagist.org/packages/techlivezheng/phpctags)
 for more details.