Include more security advices in texts

This pull request aims to add a few more contextual security advices so that some texts don't sound like users don't need to care about security.

- Mention that wallets and payment requests need to be secured (for businesses and developers).
- Explains that no PCI compliance means you don't store customers' sensitive informations.
- Recommand that people should do business with organizations with an established reputation.
- Suggest that dispute mediation services could be a future solution to provide reversible payments.