enhance installation guide with login salt recommendation and configuration details

[cosmadora]

Hi there, I enjoy your project immensly! However, I'd like to propose some changes to the documentation:

• It should be described better that it is highly recommended to set a salt before creating users
• It might be interesting for administrators to learn that it's possible to enable guest editing without guests having the ability to create or store files.

I'd appreciate your feedback.

[mathilde-cryptpad]

Hello @cosmadora,

Thanks a lot for your contribution. 👍🏻