docs: add section about creating scripts

scripts/README.md

@@ -2,7 +2,23 @@
 
 You can add custom scripts to this directory and declare them to be run at build time in the `scripts:` section of `recipe.yml`. Custom scripts can be run at either the `pre:` execution phase right after the custom repositories are added, or at the `post:` phase after all of the automatic build steps.
 
-Your scripts will be given exactly one argument when they are executed, which specifies its precise execution phase (`pre` or `post`). The primary purpose of this argument is to streamline the reuse of scripts for multiple stages. This behaviour is mirrored both in manually declared scripts and scripts ran by `autorun.sh`.
+Your scripts will be given exactly one argument when they are executed, which specifies its precise execution phase (`pre` or `post`). The primary purpose of this argument is to streamline the reuse of scripts for multiple stages. This behavior is mirrored both in manually declared scripts and scripts ran by `autorun.sh`.
+
+## Creating a script
+
+Look at `example.sh` for an example shell script. You can rename and copy the file for your own purposes. In order for the script to be executed, either move it to `scripts/pre/` or `scripts/post/` (if using `autorun.sh`) or declare it in the `recipe.yml`.
+
+All commands from programs installed as rpms should be available when running the script at the `post` execution phase.
+
+When creating a script from make sure
+
+- its filename ends with `.sh`
+  - Follows convention for (especially bash) shell scripts
+  - `autorun.sh` only executes files that match `*.sh`
+- it starts with a [shebang](<https://en.wikipedia.org/wiki/Shebang_(Unix)>) like `#!/usr/bin/env bash`
+  - Ensures the script is ran with the correct interpreter / shell
+- it contains the command `set -oue pipefail` near the start
+  - This will make the image build fail if your script fails. If you do not care if your script works or not, you can omit this line.
 
 ## `autorun.sh`