Update blog policy for posts including other CNCF projects

[svrnm]

We should consider updating our blog policy for blog posts that not only talk about OpenTelemetry but also about other CNCF projects (like Prometheus). In that case the author of the blog post should let them know (by tagging maintainers, raising an issue at the project, ping them via slack, etc.) that this blog post is work in progress and that they can inject their opinion. This is especially important when the blog post does a comparison between projects

cc @dashpole