usb: make max interface count configurable at compile time.

embassy-usb/README.md

@@ -1,6 +1,28 @@
 # embassy-usb
 
-TODO crate description/
+TODO crate description
+
+## Configuration
+
+`embassy-usb` has some configuration settings that are set at compile time, affecting sizes
+and counts of buffers.
+
+They can be set in two ways:
+
+- Via Cargo features: enable a feature like `<name>-<value>`. `name` must be in lowercase and
+use dashes instead of underscores. For example. `max-interface-count-3`. Only a selection of values
+is available, check `Cargo.toml` for the list.
+- Via environment variables at build time: set the variable named `EMBASSY_USB_<value>`. For example 
+`EMBASSY_USB_MAX_INTERFACE_COUNT=3 cargo build`. You can also set them in the `[env]` section of `.cargo/config.toml`. 
+Any value can be set, unlike with Cargo features.
+
+Environment variables take precedence over Cargo features. If two Cargo features are enabled for the same setting
+with different values, compilation fails.
+
+### `MAX_INTERFACE_COUNT`
+
+Max amount of interfaces that can be created in one device. Default: 4.
+
 
 ## Interoperability