Clarify/fix documentation about encoding for translated resource bundles.

According to http://maven.apache.org/plugins/maven-site-plugin/i18n.html translators are expected to provide their properties files in UTF-8 encoding. However, the JRE reads properties files using ISO-8859-1 encoding (see class javadoc for PropertyResourceBundle).

Either the documentation should be fixed to state ISO-8859-1 or it should be made clear why Maven requires another encoding than usual.