Overhauls `README.md` for 3.4.

Adds tutorials:
  - `minutes.ipynb`
  - `sessions.ipynb`
  - `calendar_properties.ipynb`
  - `calendar_methods.ipynb`
  - `trading_index.ipynb`

Also:
- Adds XETR and XTAE to calendars table.
- Corrects column alignment of calendar print.