Update README to include updated TCMAP & volunteer info

[mc-funk]

What

Updates README to include info on TCMAP and volunteering.

Why

Better document the bridge between the OTC and TCMAP sides, and allow people a path there that isn't word of mouth.

No code changes :)

Ensure the following interactions work as expected. Please test using a mobile form factor.

• [ ] Tapping on a marker on the map displays information about the marker in a popup.
• [ ] Tapping the "Show list of locations" button replaces the map view with a list view.
• [ ] Tapping an item in the list replaces the list view with a map view, and navigates the map to the tapped item on the map.
• [ ] Tapping one of the ✅'s in the legend filters items on the map and in the list.
• [ ] Changing the language to spanish changes things on the page.
• [ ] Clicking the Help/Info button opens and closes a menu with information.
• [ ] Clicking the X Close button on the Help/Info screen closes the modal.

Check the app in the following web browsers:

• [ ] Chrome
• [ ] Firefox
• [ ] Edge
• [ ] Safari