Add `CONTRIBUTING.md` and `CODE_OF_CONDUCT.md`

[flarnie]

In the past there was not info and support to add these docs to all Facebook Open Source projects. It's a useful measure that we can take now to create a healthy, welcoming open source community. :)

why make this change?: Facebook Open Source provides a Code of Conduct statement for all projects to follow, to promote a welcoming and safe open source community.

Exposing the COC via a separate markdown file is a standard being promoted by Github via the Community Profile in order to meet their Open Source Guide's recommended community standards.

As you can see, adding this file will improve the Facebook Ads API SDK for PHP community profile checklist and increase the visibility of our COC.

test plan: Viewing it on my branch -

issue: internal task t23481323

[flarnie]

friendly ping~

[wanleng559]

code of conduct