Code review comment for ~ssweeny/snappy-hwe-snaps/+git/tpm2:aliases

Scott, feel free to push back against this if you feel it's too much scope-creep, but could you add a basic index.md file located at docs/index.md to this MR just explaining the very basics of what tpm is, what tpm2 gives you specifically, and a very basic how to use the snap? We need to start adding the basic doc structure to all of our snaps that are missing them today.