README update

[imaginator]

When someone searches for Simplicity Blockstream this is the first code repo that shows up. Perhaps it makes sense for the README to include a brief description of how the Simplicity and SimplicityHL repos tie together and to mention the docs repo.

[apoelstra]

Yeah, we did this in https://github.com/BlockstreamResearch/rust-simplicity for similar reasons. We likely want to just add a paragraph up top linking to the Simplicity website and the repos for rust-simplicity and SimplicityHL as "places you are likely looking for if you came here looking for Simplicity".