Add contribution guide in README.

README.md

@@ -49,6 +49,26 @@ Look at the man-page (`man pacundo`) for more information.
 
 - [yay](https://github.com/Jguer/yay)
 
+## Contributing
+
+If you find any issues, feel free to report them on GitHub or send me an E-Mail
+(see my website/profile for the address). I will add these issues to my personal
+Gitea page and (unless specified otherwise) mention you as the person who found
+the issue.
+
+For patches/pull requests, if you open a PR on GitHub I will likely not merge
+directly but instead apply the patches locally (via Git patches, conserving
+authorship), push them to my Gitea repository, which will finally be mirrored to
+GitHub. However, you can save me a bit of work by just sending me the Git
+patches directly (via E-Mail).
+
+If you're looking for a way to contribute, there are a few to-do items within
+the code which you can find using `grep`:
+
+```console
+# grep -n "TODO" pacundo.pl
+```
+
 ## License
 
 This project is licensed under the terms & conditions of the Zlib license.