Explain how to contribute

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diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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@@ -0,0 +1,36 @@
+# Contributing
+
+This document explains how to contribute to this project using email.
+
+## Sending patches
+
+Please submit patches to `<patches@usebox.net>`. The preferred way to do that is [git send-email](https://git-scm.com/docs/git-send-email) (see [this step-by-step guide](https://git-send-email.io/) for more information).
+
+Alternatively, you can push your changes to a public repository, for example hosted on your own Git server, on Sourcehut, Gitlab or GitHub, and use [git request-pull](https://git-scm.com/docs/git-request-pull) to send a pull request.
+
+If these options don't work for you, just use your mail client to send a mail with a link to your changes and a short description.
+
+### Example: git send-email
+
+This example shows how to prepare a patch. First you have to check out the Git repository and configure the destination email address and the subject prefix:
+
+```
+$ git clone https://git.usebox.net/spacebeans && cd spacebeans
+$ git config sendemail.to "patches@usebox.net"
+$ git config format.subjectPrefix "PATCH SpaceBeans"
+```
+
+Then you can make and commit your changes.
+
+Finally use `git send-email` to generate a patch for the last commit and send it:
+
+```
+$ git send-email HEAD^
+```
+
+This example assumes that you have already configured [git send-email](https://git-scm.com/docs/git-send-email) for sending mails.
+
+## Contact
+
+Feel free to [contact me](https://www.usebox.net/jjm/about/me/) for questions or to discuss big changes in advance.
+
diff --git a/README.md b/README.md
index 8b318f2..d84210c 100644
--- a/README.md
+++ b/README.md
@@ -157,6 +157,8 @@ To check for dependency updates use:
 ./mill mill.scalalib.Dependency/showUpdates
 ```
 
+Please read [CONTRIBUTING](CONTRIBUTING.md) if you would like to contribute to this project.
+
 ## License
 
 Copyright (C) 2021, 2022 Juan J. Martinez <jjm@usebox.net>