[#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.

I didn't have time to fill in the section on how to i18n JavaScript though (JavaSCript, always a third-class citizen in CKAN). Also we're still just glossing over the fact tha you can mark all your strings for translation in your extension, but you can't actually extract, translate and integrate the extension translations back into CKAN.

We still need to sort out the way we use Transifex as well.