Updated Requirements (markdown)

Author: gsherwood

wiki/Requirements.md

@@ -1 +1,6 @@
-PHP_CodeSniffer requires PHP version 5.4.0 or greater, although individual sniffs may have additional requirements such as external applications and scripts. See the [[Configuration Options]] manual page for a list of these requirements.
+PHP_CodeSniffer requires PHP version 5.4.0 or greater, although individual sniffs may have additional requirements such as external applications and scripts. See the [[Configuration Options]] manual page for a list of these requirements.
+
+Additionally, PHP_CodeSniffer requires the following PHP extensions to be enabled:
+- Tokenizer: used by the core tokenzier to process PHP files
+- SimpleXML: used to process ruleset XML files
+- XMLWriter: used to create some report formats