Manual.md: add instructions for Poetry-built Python packages

Manual.md

@@ -1546,6 +1546,12 @@ dependencies at build time. If you notice warnings about `EasyInstall` deprecati
 present in `${wrksrc}/.eggs` after building the package, then those packages should be added to
 `hostmakedepends`.
 
+Python packages that use [Poetry](https://github.com/python-poetry/poetry-core)
+as a build backend should specify the `python3-pep517` build style, add
+`python3-poetry-core` to `hostmakedepends` and approximately map
+`tool.poetry.dependencies` entries in `pyproject.toml` to `depends` in the
+template.
+
 The following variables may influence how the python packages are built and configured
 at post-install time: