Added help information of plural forms

This modification to the stable version of Pootle 2.5.0 helps users who
may be unfamiliar with the concept of "Plural Forms" and what they
represent.  Most translation projects will be geared towards translators
who have some experience in development or translation, and hence will
know what Plural Forms are already.

Some projects, such as Multi Theft Auto
(http://translate.multitheftauto.com) rely on end users to provide
translations.  While they are not necessarily best placed to provide
translations, they are a useful asset for localization especially when
professional resources are not available.

This modification simply provides a link to Launchpad's translation
database which provides useful information on Plural Forms which
corresponds to those used in Pootle.  For example, when translating a
Plural string in Polish, a URL with text "Click here for information
about what each Plural Form represents" appears with a link pointing to
https://translations.launchpad.net/+languages/pl .  This provides clear
information about what the Plural Form 0/1/2 actually mean.

Future optimizations should consider localizing this actual string
rather than it always being in English.