Update README.

Author: LawnGnome

README.md

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-POSIX Regex Functions
+# POSIX regex functions
+
+## Warning
+
+This extension is deprecated and unmaintained.
+
+## Description
+
+This extension provides the ereg family of functions that were provided with
+PHP 3-5. These functions have been superseded by the preg family of functions
+provided by the PCRE extension: http://php.net/pcre
+
+Although it should be possible to build this extension with PHP 7.0, you are
+strongly encouraged to port your code to use PCRE, as this extension is not
+maintained and is available for historical reasons only.