CONTRIBUTING.md: new file

Signed-off-by: Giuseppe Scrivano <[email protected]>

CONTRIBUTING.md

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+# Contributing to crun
+
+Thanks for your interest in contributing!
+
+## Note: Before writing a big patch
+
+If you plan to contribute a large change, please get in touch *before*
+submitting a pull request by e.g. filing an issue describing your proposed
+change. This will help ensure alignment.
+
+## Background knowledge
+
+You will need to understand C to contribute to the core. There is also
+some Rust (more on this below).
+
+## Development environment
+
+Crun at its core is a low-dependency C library and CLI tools. You'll
+need a Linux environment, which could be a container or a VM/physical system.
+
+crun should be buildable on nearly any relatively modern Linux OS/distribution.
+
+### Building and testing
+
+crun uses [GNU Autotools](https://www.gnu.org/software/automake/manual/html_node/Autotools-Introduction.html) for its build system. Ensure that Autotools is installed and properly configured on your system.
+
+#### Setup
+
+To set up the build, run the following commands in the root directory:
+
+```bash
+./autogen.sh
+./configure
+make -j $(nproc)
+```
+
+
+## Testing
+
+To run the crun tests suite, you can use the following command:
+
+```bash
+make check
+```
+
+## Code linting
+
+Be sure you've run
+```
+make clang-format
+```
+
+to reformat the code automatically.
+
+## Submitting a patch
+
+The podman project has some [generic useful guidance](https://github.com/containers/podman/blob/main/CONTRIBUTING.md#submitting-pull-requests);
+like that project, a "Developer Certificate of Origin" is required.
+
+### Sign your PRs
+
+The sign-off is a line at the end of the explanation for the patch. Your
+signature certifies that you wrote the patch or otherwise have the right to pass
+it on as an open-source patch. The rules are simple: if you can certify
+the below (from [developercertificate.org](https://developercertificate.org/)):
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+660 York Street, Suite 102,
+San Francisco, CA 94110 USA
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+Then you just add a line to every git commit message:
+
+    Signed-off-by: Joe Smith <[email protected]>
+
+Use your real name (sorry, no pseudonyms or anonymous contributions.)
+
+If you set your `user.name` and `user.email` git configs, you can sign your
+commit automatically with `git commit -s`.
+
+### Git commit style
+
+Please look at `git log` and match the commit log style, which is very
+similar to the
+[Linux kernel](https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git).
+
+Then you just add a line to every git commit message:
+
+    Signed-off-by: Joe Smith <[email protected]>
+
+Use your real name (sorry, no pseudonyms or anonymous contributions).
+
+1. Title
+    - Specify the context or category of the changes e.g. `lib` for library changes, `docs` for document changes, `bin/<command-name>` for command changes, etc.
+    - Begin the title with the first letter of the first word capitalized.
+    - Aim for less than 50 characters, otherwise 72 characters max.
+    - Do not end the title with a period.
+    - Use an [imperative tone](https://en.wikipedia.org/wiki/Imperative_mood).
+2. Body
+    - Separate the body with a blank line after the title.
+    - Begin a paragraph with the first letter of the first word capitalized.
+    - Each paragraph should be formatted within 72 characters.
+    - Content should be about what was changed and why this change was made.
+    - If your commit fixes an issue, the commit message should end with `Closes: #<number>`.
+
+Commit Message example:
+
+```bash
+<context>: Less than 50 characters for subject title
+
+A paragraph of the body should be within 72 characters.
+
+This paragraph is also less than 72 characters.
+```
+
+For more information see [How to Write a Git Commit Message](https://chris.beams.io/posts/git-commit/)