Update README.md

[cdeil]

This PR is a follow-up to #3 and contains some small changes to the README:

• Mention what is available and link to API docs at the top
• Also put install command at the top, instead of after the longish plotting example
• Remove the Progress table. It doesn't contain extra info compared to the API docs, and manually keeping the table in sync with the API docs is tedious / error-prone.

[michitaro]

Thanks!