[#1573] Add new translations policy to docs
[#809] Add i18n guide for devs

Fixes #809
[#1564] Add beginner contribution docs on 'Good for Contribution'
Merge pull request #1628 from ckan/809-string-i18n-docs

[#809] Add i18n guide for devs
Merge branch '1573-new-translations-policy'
Merge branch 'master' of github.com:ckan/ckan