Update README with links to docs and new dependencies

[philbull]

No description provided.

[coveralls]

Coverage increased (+0.1%) to 92.108% when pulling 13cbfc9 on readme-update-docs into ec8b94a on master.

[nkern]

ooh, great. our README is severely out of date. Can you add to the end of the README something along the lines of

## Running `hera_pspec`

See [`examples/PS_estimation_examples.ipynb`](link) for a brief tutorial on how to create delay spectra with the various modules in `hera_pspec`.

See [`examples/PSpecBeam_tutorial.ipynb`](link) for a brief tutorial on handling beam objects in `hera_pspec`.

[philbull]

Done. Thanks!