Strengthen AI guidlines & force screenshots for visual users

IMHO after https://github.com/PrivateBin/PrivateBin/pull/1754 we again need to make it a requirement in the guidelines to disclose the fact an LLM is used.

This also adds to attach screenshots for visual changes etc. (which again also ensures users actually _test_ the changes they submit)

I wrote it here and not in the wiki, because:
* this text can be read by an LLM inside the git repo, which I guess is beneficial
* the text may be shown more prominently ere IMHO
* I guess the wiki guidelines rather give a "big introduction" into how to contribute with the code base, and do not define "rules"/guidelines at all

.github/CONTRIBUTING.md

@@ -6,3 +6,10 @@ Have a look at our [contributing guide](https://github.com/PrivateBin/PrivateBin
 If you want to translate PrivateBin into your language have a look at the [translation guide](https://github.com/PrivateBin/PrivateBin/wiki/Translation).
 
 Except this also opening [issues](https://github.com/PrivateBin/PrivateBin/issues) helps much. Just describe your problem detailed enough and fill out our template.
+
+## Guidelines for pull requests
+
+Please note that we, as per our pull request template, **require users to disclose the use of an AI/LLM tool**. We would be glad about details such as the exact used tool/relevant chat snippets or a full (link to the) chat conversation. In any case, please take care to manually test and review your pull request.
+
+For Frontend adjustments or other changes visible visually in the PrivateBin web UI, please _always_ attach at least **a screenshot** of how it loos like with your changes applied. You may only omit that for invisible changes or _very_ obvious little changes like fixing typographic mistakes or translations etc.
+If possible, especially for bigger or interactive changes, please also attach a link to a working test instance.