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!
list all maintainers in the readme
change maintainer list

I don't know who I am
Implement flash_helper to remove unwanted keys from flash. Fixes #3316
Merge pull request #3507 from quikly/flash-helper

Implement flash_helper to remove unwanted keys from flash. Fixes #3316
Update 5-forms.md

makes nested resources more clear for noobs like me.
Name action items; allow removing them by name

Ref #760

Name first parameter to action items.
It is still optional in the DSL, but triggers a deprecation warning when not provided.
Merge pull request #3091 from carnesmedia/name-action-items

Name action items; allow removing them by name
suppress and spec deprecation warning in dsl_spec
Refactor FormBuilder#has_many by extracting methods.
Refactor FormBuilder, replacing form_buffers with Arbre components.  See PR #3486 for discussion.
Remove form_buffer from filters form.
Fix active_admin_comments now that active_admin_form_for is a nested Arbre component.
Fix new password form, we can no longer break up active admin form in an ERB template.
Remove duplicate copy of test.
Formtastic + Arbre: will it blend?
Update documentation for forms including Arbre.
Merge pull request #3486 from varyonic/2542-arbe-buffering-inside-aa-forms

Convert active_admin_form_for to Arbre component - resolves #2542
on table index column block add sortable column by default
add #3075 to changelog