Document serializers

Description

We recently introduced app/serializers, but we forgot about documentation on how to use this new concept.

Proposal

Document serializers in app/serializers/README.md, just like we will soon have app/presenters/README.md.

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/issues/23886, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8480, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7174

/cc @rymai @rspeicher