Adding more README's for the subdirectories

I am a beginner Rust programmer and wanted to try and contribute code to AB-street

However, I am very confused on the role of each subdirectory and how I can begin to understand the functionality of each component of AB-street

This Is very likely mainly due to my inexperience with the software and more importantly the language itself. However, it would be nice to have some sort of guidelines on what is happening in each subdirectory.

Currently these are the directories that already have READMEs

Tasklist for remaining crates:
- [ ] abstio
- [ ] cloud
- [x] geom
- [ ] map_gui
- [ ] raw_map
- [ ] target
- [ ] updater
- [ ] abstutil
- [ ] collisions
- [ ] headless
- [ ] map_model
- [ ] release
- [ ] tests
- [x] web
- [x] apps
- [ ] convert_osm
- [ ] importer
- [x] piggyback
- [ ] sim
- [ ] traffic_seitan
- [x] widgetry
- [ ] cli
- [x] data
- [ ] kml
- [ ] popdat
- [ ] synthpop
- [ ] traffic_signal_data
- [ ] widgetry_demo
- [ ] update the corresponding docs [section](https://a-b-street.github.io/docs/tech/dev/index.html#code-organization)