Add to the documentation how to encode % sign

[Smolky]

I have noticed that translating characters which contains "&" sign should be properly scaped

Example: \"%word1%,word2%\"

to output "%word1%,word2%

But maybe this information will be useful in the docs

[Philipp15b]

HTML escaping is not in php-i18n's scope, but I will accept merge requests to add some documentation for this.