Add a guide for third-party exercises

Author: mo8it

README.md

@@ -117,11 +117,15 @@ See the footer of the list for all possible keys.
 
 ## Continuing On
 
-<!-- TODO: Mention third-party exercises -->
-
 Once you've completed Rustlings, put your new knowledge to good use!
 Continue practicing your Rust skills by building your own projects, contributing to Rustlings, or finding other open-source projects to contribute to.
 
+## Third-Party Exercises
+
+Do you want to create your own set of Rustlings exercises to focus on some specific topic?
+Or did you want to translate the original Rustlings exercises?
+Then follow the link to the guide about [third-party exercises](THIRD_PARTY_EXERCISES.md)!
+
 ## Uninstalling Rustlings
 
 If you want to remove Rustlings from your system, run the following command:

THIRD_PARTY_EXERCISES.md

@@ -0,0 +1,53 @@
+# Third-Party Exercises
+
+The support of Rustlings for third-party exercises allows you to create your own set of Rustlings exercises to focus on some specific topic.
+You could also offer a translatation of the original Rustlings exercises as a third-party exercises.
+
+## Getting started
+
+To create third-party exercises, install Rustlings and run `rustlings dev new PROJECT_NAME`.
+This command will, similar to `cargo new PROJECT_NAME`, create a template directory called `PROJECT_NAME` with all what you need to get started.
+
+Read the comments in the generated `info.toml` file to understand the format of this file.
+It allows you to set a custom welcome and final message and specify the metadata of every exercise.
+
+## Create an exercise
+
+Here is an example of the metadata of one file:
+
+```toml
+[[exercises]]
+name = "intro1"
+hint = """
+To finish this exercise, you need to …
+This link might help you …"""
+```
+
+After entering this in `info.toml`, create the file `intro1.rs` in the `exercises/` directory.
+The exercise needs to contain a `main` function, but it can be empty.
+Adding tests is recommended.
+Look at the official Rustlings exercises for inspiration.
+
+You can optionally add a solution file `intro1.rs` to the `solutions/` directory.
+
+Now, run `rustlings dev check`.
+It will tell you about any issues with your exercises.
+For example, it will tell you to run `rustlings dev update` to update the `Cargo.toml` file to include the new exercise `intro1`.
+
+`rustlings dev check` will also run your solutions (if you have any) to make sure that they run successfully.
+
+That's it!
+You finished your first exercise 🎉
+
+## Publish
+
+Now, add more exercises and publish them as a Git repository.
+
+Users just have to clone that repository and run `rustlings` in it to start working on your set of exercises just like the official ones.
+
+One difference to the official exercises is that the solution files will not be hidden until the user finishes an exercise.
+But you can trust the user to not look at the solution too early ;)
+
+## Share
+
+After publishing your set of exercises, open a pull request in the official Rustlings repository to link to your project in the README 😃