Add info about contributing to docs

CONTRIBUTING.md

-# How to submit a Pull Request to AFL++
+# Contributing to AFL++
+
+## How to submit a pull request
 
 All contributions (pull requests) must be made against our `dev` branch.
 
@@ -20,4 +22,38 @@ and use AFL's macros wherever possible (e.g., WARNF, FATAL, MAP_SIZE, ...).
 
 Remember that AFL++ has to build and run on many platforms, so generalize your
 Makefiles/GNUmakefile (or your patches to our pre-existing Makefiles) to be as
-generic as possible.
+generic as possible.
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+
+## How to contribute to the docs
+
+We welcome contributions to our docs.
+
+Before creating a new file, please check if your content matches an existing
+file in one the following folders:
+
+* [docs/](docs/) (this is where you can find most of our docs content)
+* [frida_mode/](frida_mode/)
+* [instrumentation/](instrumentation/)
+* [qemu_mode/](qemu_mode/)
+* [unicorn_mode/](unicorn_mode/)
+
+When working on the docs, please keep the following guidelines in mind:
+
+* Edit or create Markdown files and use Markdown markup.
+  * Do: fuzzing_gui_program.md
+  * Don't: fuzzing_gui_program.txt
+* Use underscore in file names.
+  * Do: fuzzing_network_service.md
+  * Don't: fuzzing-network-service.md
+* Use a maximum of 80 characters per line to make reading in a console easier.
+* Make all pull requests against `dev`, see
+  [#how-to-submit-a-pull-request-to-afl](#how-to-submit-a-pull-request-to-afl).
+
+And finally, here are some best practices for writing docs content:
+
+* Use clear and simple language.
+* Structure your content with headings and paragraphs.
+* Use bulleted lists to present similar content in a way that makes it easy to
+  scan.
+* Use numbered lists for procedures or prioritizing.
+* Link to related content, for example, prerequisites or in-depth discussions.
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