Added deprecation notice to README
Updating README with deprecation notice so that if team members come to this project they are aware of the status and pointer to IMA work (and the relevant deprecation issue).
Codebase for IMA: https://ops.gitlab.net/gitlab-com/gl-infra/incident-management
Anthony / Andrew - Could one of you review / approve please?
Edited by Amarbayar Amarsanaa