Document preferred tag list in Blog README

@Thingee has kindly been doing the lift on exporting legacy blog posts from the existing site and importing into the new site (#116).

Along with that comes a refinement in how we tag blog posts for the site. The existing site has an extensive (potentially too extensive) list of tags used: https://ceph.io/community/blog/

Going forward, we've proposed we'll focus this list in, based on this [proposed taglist](https://github.com/ceph/ceph.io/files/6637960/proposed-taglist.txt), to provide more relevant and helpful blog category listings. Other tags ([here for posterity](https://github.com/ceph/ceph.io/files/6637968/removed-taglist.txt)) will no longer be encouraged.

We should add the proposed taglist to the [Blog's `README`](https://github.com/ceph/ceph.io/tree/develop/src/en/news/blog#readme) file, so newcomers and blog contributors are guided to use the most relevant tags.

Open the Blog README at src/en/news/blog and review the proposed taglist linked in the issue. Add the proposed tags so blog contributors can choose the preferred list, while retaining the README's existing guidance. Done means the README documents the proposed tag list clearly for newcomers.

Written by the indexing model from the issue text.