Add warning about CVE-2013-6393 to CHANGELOG [ci skip]

Actually resolves https://github.com/sstephenson/ruby-build/issues/499.

CHANGELOG.md

 ## Version History
 
 #### 20140204
-* Update libyaml to version 0.1.5
 * Add Rubinius 2.2.4
+* Update libyaml to version 0.1.5
+
+Note: If Psych::LIBYAML_VERSION < '0.1.5' you should update and rebuild Ruby.
+For more informaton see CVE-2013-6393.
 
 #### 20140110.1
 * Fix bug in JRuby 1.7.10 definition