Add draft of Python modules primer

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+# Python Modules Primer
+
+## Prerequisites
+
+These instructions assume a Linux environment.
+A macOS environment is similar, but not identical.
+A Windows environment is more different.
+
+## Previous knowledge
+
+### A refresher on the `PATH` variable
+
+If you execute the following command in your terminal:
+
+```
+$ echo hello
+```
+
+, the shell searches for the `echo` command in the directories listed in your `PATH` environment variable.
+You can display your `PATH` variable by running:
+
+```
+$ echo $PATH
+/home/user/.local/bin:/home/user/bin:/usr/share/Modules/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin
+```
+
+The contents of the `PATH` variable depend on your particular environment.
+
+If you run the following command:
+
+```
+$ which echo
+/usr/bin/echo
+```
+
+The `which` command prints where the shell locates the `echo` command.
+
+### A refresher on shell scripts
+
+If you create a file named `foo.sh` with the following contents:
+
+```
+#!/bin/sh
+
+echo hello
+```
+
+You define a "shell script".
+The first line indicates that this shell script is executed by using the `/bin/sh` command.
+The rest of the file are commands to be executed by the shell command.
+These commands behave as if you typed them into your terminal, so if you execute this script, the command `echo hello` will be executed, printing `hello`.
+
+If you try to run `foo.sh` like you run the `echo` command, by typing its name, it does not work:
+
+```
+$ foo.sh
+bash: foo.sh: command not found...
+```
+
+, because the shell looks for the `foo.sh` in the directories listed in the `PATH` variable.
+Unless you created the `foo.sh` file in a directory like `/usr/bin`, the shell will not find the `foo.sh` command.
+
+A solution to this problem is to specify the path to the `foo.sh` file, instead of relying on the `PATH` variable.
+However, if you do this, you face a second problem.
+
+```
+$ ./foo.sh
+bash: ./foo.sh: Permission denied
+```
+
+This happens because only files with the executable permission can be executed in this way.
+To solve this, add the executable permission; then it works:
+
+```
+$ chmod +x foo.sh
+$ ./foo.sh
+hello
+```