generic: add comment about the new design

The bit about redesigning could be removed after it's finished, but I'd
leave the principles and maybe even add some information for aspiring
contributors.

avr-hal-generic/src/lib.rs

@@ -1,6 +1,38 @@
 #![no_std]
 #![cfg_attr(avr_hal_asm_macro, feature(asm_experimental_arch))]
 #![cfg_attr(not(avr_hal_asm_macro), feature(llvm_asm))]
+//! Defines the internal API used to generate the per-device external API
+//! (user-facing) in the HAL crates.
+//!
+//! # Macro Design (WIP)
+//!
+//! As part of an effort to make the macros here more maintainable and less
+//! repetitive, they are being restructured module-by-module. Currently revised
+//! modules are:
+//! - [`adc`]
+//! - [`port`]
+//!
+//! The general guiding principals are:
+//! 1. What goes inside the macro invocation should look like regular code as
+//! much as possible;
+//! 2. Information related to groups of implementations of a feature should be
+//! encoded as alternative matchers in the macro, rather than by introducing
+//! many metavariables that each invocation will need to repeat;
+//!    As an example of such information, take the ADC's reference voltage.  All
+//! Atmega processors can be abstracted with the same definition of the
+//! `ReferenceVoltage` type, but Attiny processors differ among themselves and
+//! also from the Atmega implementation. Rather than leave that type up to the
+//! invocation, write one fully general matcher and write smaller matchers that
+//! expand to pre-filled versions of the former. The HAL crates then use these
+//! as much as possible, falling back only when there is singular hardware
+//! that would need its own matcher but would use it only once.
+//! 3. Information unique to each implementation should be left to the
+//! invocation, but make the macro smart enough to avoid repeating ourselves.
+//!    An example of this is the mapping between ADC channels and pins. The best
+//! scenario here is a mapping like `<channel> = <pin>`, maybe `<channel>:
+//! <type> = <pin>`, if there's more information needed to encode the mapping.
+//! [`paste::paste`] can be used for gluing the information into adequate
+//! identifiers.
 
 pub use embedded_hal_v0 as hal;