[documentation] Update README with usage examples for crypto endpoint

## Motivation

- We document all the main ticker price endpoints in the README, but we don't have example for crypto functions. Thanks @sher85 for noticing this in https://github.com/hydrosquall/tiingo-python/pull/340#issuecomment-705245418 .

## Subtasks

- [x] Update README https://github.com/hydrosquall/tiingo-python/blob/master/README.rst
- [ ] Update USAGE.rst
- [ ] REACH - Add a sample jupyter notebook that interacts with the crypto endpoints to show a practical usecase, in the `/examples` [folder](https://github.com/hydrosquall/tiingo-python/tree/master/examples). Visitors will be able to visit example using a binder link like this: https://mybinder.org/v2/gh/hydrosquall/tiingo-python/master?filepath=examples

## Resources
- The official API documentation https://api.tiingo.com/documentation/crypto
- The PR which added the crypto endpoint functions: https://github.com/hydrosquall/tiingo-python/pull/340, which contains test code showing how each function can be called (https://github.com/n1rna/tiingo-python/blob/fb1295376a1049b8d2d8819a648b82c9f7c0087b/tests/test_crypto.py#L19-L37 )