docs: add more about downstream (#347)

Adding some docs about downstream usage.

---------

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

.github/CONTRIBUTING.md

@@ -1,3 +1,20 @@
+# Testing a project with a branch / main
+
+If you are testing a downstream project, you can use a branch of
+scikit-build-core like this:
+
+```toml
+[build-system]
+requires = ["scikit-build-core @ git+https://github.com/scikit-build/scikit-build-core@main"]
+build-backend = "scikit_build_core.build"
+```
+
+Or you can build your project from the scikit-build-core source with nox:
+
+```bash
+nox -s downstream -- https://github.com/...
+```
+
 # Setting up for development
 
 See the [Scikit-HEP Developer introduction][skhep-dev-intro] for a detailed

docs/cmakelists.md

@@ -26,7 +26,7 @@ If you are making a Limited ABI / Stable API package, you'll need the
 `Development.SABIModule` component instead. You can use the
 `SKBUILD_LIMITED_API` variable to check to see if it was requested.
 
-If you want to use the old, deprecated FindPythnoInterp and FindPythonLibs
+If you want to use the old, deprecated FindPythonInterp and FindPythonLibs
 instead, you can. Though it should be noted that FindPythonLibs requires a trick
 to make it work properly if a Python library is not preset (like in manylinux):
 you have to set `PYTHON_LIBRARY` to something (doesn't matter what) to make it

noxfile.py

@@ -154,7 +154,7 @@ def downstream(session: nox.Session) -> None:
     else:
         import tomllib
 
-    parser = argparse.ArgumentParser()
+    parser = argparse.ArgumentParser(prog=f"{Path(sys.argv[0]).name} -s downstream")
     parser.add_argument("project", help="A project to build")
     parser.add_argument("--subdir", help="A subdirectory to build")
     args, remaining = parser.parse_known_args(session.posargs)