Adds review app documentation to the readme
Thanks @northrup
This is essentially a test if the review apps are working! With this I added a notion of review apps to the readme section.
Closes https://gitlab.com/gitlab-com/infrastructure/issues/3763 https://gitlab.com/gitlab-org/design.gitlab.com/issues/50
Ps: @northrup has opened another issue with CI as it seems that there can be a slowness in the CI execution, @Kamil has said he’s going to look into it on Monday.