Allow for possiblity that randomness cannot be controlled for in section on Randomness

[larsvilhuber]

https://github.com/social-science-data-editors/template_README/blob/47ed05565b53b0db5813c7973ad44fba7b075943/templates/README.md?plain=1#L182

This currently only has two options:

• there is randomness, but we can control the PseudoRNG
• there is no randomness

There are a few scenarios where it is not possible to fully control for randomness, and that is known ex ante. Add an option for that.