Add documentation to the semantic convention YAML models directory

[justinfoote]

I found the YAML files in the semantic_conventions directory at the root of this repository to be something of a mystery. When I first discovered them, I thought they were duplicates of the semantic conventions found in markdown in the specs. It took quite a bit of digging to find this pr and issue and subsequently this build tool, and now I've used the docker build tool to generate semantic convention markdown.

I propose that we add a README to the main directory of the semantic conventions with links out to the build tools, so the purpose of these files will be more obvious to newcomers.