README updates

Author: lucascolley

Tools/README

@@ -43,6 +43,8 @@ patchcheck      Tools for checking and applying patches to the Python source cod
 
 peg_generator   PEG-based parser generator (pegen) used for new parser.
 
+pixi-packages   Pixi package definitions for downstream from-source builds
+
 scripts         A number of useful single-file programs, e.g. run_tests.py
                 which runs the Python test suite.
 

Tools/pixi-packages/README

@@ -0,0 +1,36 @@
+## CPython Pixi Packages
+
+This directory contains definitions for [Pixi packages](https://pixi.sh/latest/reference/pixi_manifest/#the-package-section)
+which can be built from the CPython source code.
+
+Downstream developers can make use of these packages by adding them as git dependencies in a
+[Pixi workspace](https://pixi.sh/latest/first_workspace/), like:
+
+```toml
+[dependencies]
+python = { git = "https://github.com/python/cpython", subdirectory = "Tools/pixi-packages/asan" }
+```
+
+This is particularly useful when developers need to build CPython from source
+(for example, for an ASan-instrumented build), as it does not require any manual
+cloning or building steps. Instead, Pixi will automatically handle both the building
+and installation of the package.
+
+Each package definition is contained in a subdirectory, but they share the build script
+`build.sh` in this directory. Currently defined package variants:
+
+- `default`
+- `asan`: ASan-instrumented build with `PYTHON_ASAN=1`
+
+### Maintenance
+
+- the `version` fields in each `recipe.yaml` should be kept up to date with the Python version
+- dependency requirements should be kept up to date in each `recipe.yaml`
+- `build.sh` should be updated for any breaking changes in the `configure` and `make` workflow
+
+### Opportunities for future improvement
+
+- more package variants (e.g. TSan, UBSan)
+- support for Windows
+- using a single `pixi.toml` and `recipe.yaml` for all package variants is blocked on https://github.com/prefix-dev/pixi/issues/4599
+- a workaround can be removed from the build script once https://github.com/prefix-dev/rattler-build/issues/2012 is resolved