Update README

Update code overview to reflect latest architectural changes and mention
that we need the latest stable rust compiler for building this project.

README.md

@@ -57,6 +57,7 @@ the in-game world and a custom state-machine for managing the overall gamestate.
 - `src/states/`: Game-states like _Heaven_, _In-Game_, _Main-Menu_, etc.  The
   most important one is probably `src/states/ingame.rs` which is where all the
   ECS magic happens.
+- `src/angel_shifts/`: Implementations of the different angel shifts.
 
 #### `rust-web-modules/` - TypeScript helpers for Rust
 Some things are just super ugly to do in Rust directly (anything related to the
@@ -69,10 +70,13 @@ work.
 - `www/src/_rc3-*`: Styles and Fonts for RC3
 
 #### `www/resources/` - Sprites and Levels
-- TODO
+- `www/resources/levels/` - Maps, see [Level Design](#level-design) for all the
+  gory details.
+- `www/resources/sprites/` - Sprites for in-game entities.  Sprites are loaded
+  via entries in `src/sprites.rs`.
 
 ## Build Dependencies
-- `rustc` & `cargo` (duh ... best install them via [`rustup`])
+- `rustc` & `cargo` **latest stable** (=1.48.0 as of now).  Best install them via [`rustup`] ...
 - [`wasm-pack`]
 - `npm`
 - (optional: [`cargo-watch`])