Documented the `:optional` option for belongs_to

I found the `:optional` option very useful for a project to expose nested resources in the global menu ; here's a short proposal to expose it in the documentation.

Thanks for all the good work in activeadmin, you rock!
Use the latest release of yard
Merge pull request #3522 from strivedi183/bump-yard

Use the yard 0.8.7.6 or greater
pimp the changelog

just run `pimpmychangelog`
add locale key active_admin.devise.links.sign_up
stash
add .localeapp to gitignore
Merge pull request #3472 from Bastes/patch-1

Documented the `:optional` option for belongs_to
Added tabs option for organizing content display on active admin pages.
Merge pull request #3388 from yagudaev/master

Added tabs dsl for organizing content on active admin pages
Update to the newest InheritedResources

Allow inherited resources 1.4.x
Merge pull request #3538 from petergoldstein/feature/ih_1_5_1

Update to the newest inherited_resources
Test and fix rendering of multiple input fields in has_many.