Add a section explaining the risks involved with f-sidechains for users

[wojake]

https://xrpl.org/federated-sidechains.html: Considering the amount of risks involved with federated sidechains for users:

• scams
• compromised federators
• bugs:hacks
• centralized sidechains

I suggest there should be a section in the documentation explaining the risks involved with federated sidechains for XRPL users.

Since a sidechain's door account is being handled by its federators (in a sense: custodians), it's not entirely trustless in my understanding. There's currently an amendment that increases the multisig list from 8 to 32, this would reduce these risks but the risk should still be laid out to users.