Readme: Recommend/mention dot prefixed config files (PHPCS 3.1.1)
Readme: Recommend/mention dot prefixed config files (PHPCS 3.1.1)

Since PHPCS 3.1.0, the config file can be dot-prefixed, i.e. `.phpcs.xml` or `.phpcs.xml.dist`.
Since PHPCS 3.1.1, the loading order has been corrected for the config files.

So, as the minimum PHPCS version is now well passed those versions, let's mention this in the readme.

Refs:
* squizlabs/PHP_CodeSniffer 1566
* squizlabs/PHP_CodeSniffer 1693

WordPress-Coding-Standards/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards