Document preferred tag list in Blog README

[adamduncan]

@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, to provide more relevant and helpful blog category listings. Other tags (here for posterity) will no longer be encouraged.

We should add the proposed taglist to the Blog's README file, so newcomers and blog contributors are guided to use the most relevant tags.

[Pete-Robelou]

@adamduncan Do you want me to update the blog README now or shall we wait for a first pass of the blog posts?

[adamduncan]

Happy to wait until the Blog posts land. Not an urgent one. Just one to capture prior to launch 👍 Thanks