DOC: add contributing guidelines

[ci skip]

Author: rgommers

CONTRIBUTING.md

@@ -0,0 +1,77 @@
+# Contributing to the `numpy-release` repository
+
+This repository has fairly strict contribution rules for security and
+auditability reasons, as explained in the README. PRs with improvements or bug
+fixes are very welcome, however CI jobs will not run for anyone who doesn't
+have commit access.
+
+
+## Running CI jobs on your own fork
+
+To get CI to run on your own fork for changes in a branch named
+`my-branch-name`, add a temporary commit to your branch that adds a trigger:
+
+```diff
+--- a/.github/workflows/wheels.yml
++++ b/.github/workflows/wheels.yml
+@@ -22,6 +22,7 @@ on:
+   push:
+     branches:
+       - main
++      - my-branch-name
+   workflow_dispatch:
+     inputs:
+       environment:
+```
+If you title the commit, e.g., `DEBUG: run on fork`, it's easy to drop the
+commit again once you're done testing and before opening a PR to the
+`numpy/numpy-release` repository.
+
+Note that this will run *a lot of jobs*. If you're doing iterative testing,
+it's recommended to only select the platform(s) you're interested in like this:
+
+```diff
+--- a/.github/workflows/wheels.yml
++++ b/.github/workflows/wheels.yml
+@@ -22,6 +22,7 @@ on:
+   push:
+     branches:
+       - main
++      - my-branch-name
+   workflow_dispatch:
+     inputs:
+       environment:
+@@ -48,20 +49,8 @@ jobs:
+         # Github Actions doesn't support pairing matrix values together, let's improvise
+         # https://github.com/github/feedback/discussions/7835#discussioncomment-1769026
+         buildplat:
+-          - [ubuntu-22.04, manylinux_x86_64, ""]
+-          - [ubuntu-22.04, musllinux_x86_64, ""]
+-          - [ubuntu-22.04-arm, manylinux_aarch64, ""]
+           - [ubuntu-22.04-arm, musllinux_aarch64, ""]
+-          - [macos-13, macosx_x86_64, openblas]
+-
+-          # targeting macos >= 14. Could probably build on macos-14, but it would be a cross-compile
+-          - [macos-13, macosx_x86_64, accelerate]
+-          - [macos-14, macosx_arm64, openblas]
+-          - [macos-14, macosx_arm64, accelerate]
+-          - [windows-2022, win_amd64, ""]
+-          - [windows-2022, win32, ""]
+-          - [windows-11-arm, win_arm64, ""]
+-        python: ["cp311", "cp312", "cp313", "cp313t", "cp314", "cp314t", "pp311"]
++        python: ["cp314", "cp314t"]
+         exclude:
+           # Don't build PyPy 32-bit windows
+           - buildplat: [windows-2022, win32, ""]
+```
+
+
+## Commit messages and linear history
+
+Please use the same [commit message format as for the main `numpy` repository](https://numpy.org/devdocs/dev/development_workflow.html#writing-the-commit-message).
+
+This repository requires linear history. It's preferred that contributors edit
+their commit history so the PRs they submit contain clean, independent commits.
+Note that each commit should be able to pass CI - if one commit depends on
+another, they should be merged. Maintainers may decide to squash-merge if those
+requirements aren't met.