Added Contentrain Git Based Headless CMS

[ABB65]

Contentrain is a Git-based Headless CMS that was focused on developer and content editor experience which provide seamless integration with Jamstack web apps.

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[ABB65]

Hi @hexagoncircle We are still waiting for the PR I opened in August 2022. In this time I see new CMS's listed even 2 days ago. If there is a problem with the PR we have opened, we are waiting for you to inform us. If there is no obstacle for you to publish, I understand that you are not as fast as you were for Decap, but it is not fair for us to wait for 9 months. I ask for your support. Kind regards.

[hexagoncircle]

Hi @ABB65! Thanks for bringing this PR my attention and for your patience; I agree that's too long to wait. I've very recently taken on the task of going through all open PRs on this repo when I have time. Everything here is looking good 👍