Added project structure to README

README.md

@@ -1,3 +1,19 @@
 # rust-api
 
-An API written in Rust, which I used to learn the language.
+An API written in Rust, which I used to learn the language.
+
+## Project Structure
+
+The folder structure follows the structure of the URLs, e.g. the route for
+`/hello/world` is found in the module `src/hello`.
+
+Each module contains the following base files:
+
+* `mod.rs`: defines the modules' content, and contains the route definitions.
+  The route functions themselves only contain the functionality needed to
+  represent the data, not acquire it.
+* `controller.rs`: this file contains the actual logic of each route. If the
+  logic becomes too complicated to be contained inside a single file,
+  `controller.rs` becomes its own module folder named `controller`.
+* `tests.rs`: this contains tests for the specific module. This can also be a
+  module directory if need be.