Document best practices when writing a script

[rvansa]

• general comment
• list of accepted parameters as comment after the name of the script
• if the script repeats variable with default, isolate this into initial - set-state: FOO ${{FOO:bar}}
• ... ?

[rvansa]

Also mention that passwords should not be in the scripts in plaintext, but loaded from file that could be encrypted via git crypt (or passed via the state):

• how to pass a string from file as cmdline parameter
• how to pass a string from file to command that expects the password as an interactive input

Maybe ^ shouldn't be in the same page, but should be linked.