Add contributing guide

Signed-off-by: Wiktor Kwapisiewicz <[email protected]>

CONTRIBUTING.md

@@ -1,5 +1,76 @@
 # Contributing
 
+Thanks for taking the time to contribute to this project!
+
+All changes need to:
+
+- pass basic checks, including tests, formatting and lints,
+- be signed-off.
+
+## Basic checks
+
+We are using standard Rust ecosystem tools including `rustfmt` and `clippy` with one minor difference.
+Due to a couple of `rustfmt` features being available only in nightly (see the `.rustfmt.toml` file) nightly `rustfmt` is necessary.
+
+All of these details are captured in a `.justfile` and can be checked by running [`just`'](https://just.systems/).
+
+To run all checks locally before sending them to CI you can set your git hooks directory:
+
 ```sh
 git config core.hooksPath scripts/hooks/
 ```
+
+## Developer Certificate of Origin
+
+The sign-off is a simple line at the end of the git commit message, which certifies that you wrote it or otherwise have the right to pass it on as a open-source patch.
+
+The rules are pretty simple: if you can [certify the below][DCO]:
+
+```
+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 saying
+
+    Signed-off-by: Random J Developer <[email protected]>
+
+using your name.
+
+If you set your `user.name` and `user.email`, you can sign your commit automatically with [`git commit --signoff`][GSO].
+
+To sign-off your last commit:
+
+    git commit --amend --signoff
+
+[DCO]: https://developercertificate.org
+[GSO]: https://git-scm.com/docs/git-commit#git-commit---signoff
+
+If you want to fix multiple commits use:
+
+    git rebase --signoff main
+
+To check if your commits are correctly signed-off locally use `just check-commits`.