docs: Some notes on developing C modules

Author: jonnor

doc/developing-c-modules.md

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+
+# Developing C modules for MicroPython
+
+This is a sketch for a presentation about developing
+third-party libraries for MicroPython as C modules.
+
+## Outline
+
+- External C modules, what is it, how does it work.
+- Native .mpy modules, what is it, how does it work.
+Limitations. Fundamental and incidental/current.
+- Supporting both as external C module and native.
+Some #ifdefs and duplication needed. Potential for unified macros?
+- CMake and Makefile setups for external C modules
+- Writing automated tests.
+Mostly like for .py files. pytest-style, use asserts with message
+- Running automated tests against hardware.
+Using mpremote mount. Modify sys.path for architecture specifics
+- Continious Integration setup.
+Using Github Actions. Can reuse ci shell scripts from MicroPython repo.
+- Continious Delivery setup.
+Using Github Actions -> Github pages. .mpy files
+- Supporting browser using Webassembly.
+Standard external C modules build.
+- Supporting running on PC
+! Native modules do not work in Mac OS.
+! Windows only supported using Windows Subsystem for Linux?
+- API refererence documentation.
+Using stub files sphinx with sphinx-autodoc-typehints
+- ?? How to use the stubs for typechecking, like micropython stubs
+
+## Examples
+
+- emlearn-micropython, https://github.com/emlearn/emlearn-micropython/
+- tamp, https://github.com/BrianPugh/tamp/tree/main/mpy_bindings
+- ulab, https://github.com/v923z/micropython-ulab