[#809] Add i18n guide for devs

Fixes #809. This is in response to an old email from a translator: <https://lists.okfn.org/pipermail/ckan-dev/2013-April/004563.html>, it's been assigned to me for ages and I've finally done it.

Now if we can sort out the way we use Transifex, we'll finally have pretty decent i8n and l10n processes (and then we can try to do good in practice as well)