Note on macOs installation

[maciej-radziejewski]

Affects versions :

• OS: Mac
• Version of Xournal++: docs

Describe the bug
You suggested to improve the macOs installation guide, so here is a proposed change.

"Just download the archive and run the unzipped executable.”
⟶

1. Download the archive.
2. Move the unzipped executable to the Apps folder.
3. Right-click the executable and choose "Open”.
4. Read the prompt and confirm (if you want to use the program).
5. Once confirmed, you can run it like other programs.

To Reproduce
If you just double-click the executable the first time, macOs will not run it.

[LittleHuba]

I'll move the discussion over to the PR.