Add note about fee collection

[noandrea]

No description provided.

[github-actions]

Coverage report

Filename	Lines Covered (%)	Functions Covered (%)
order-engine-sdk/src/fill.rs	0	0
order-engine-sdk/src/lib.rs	0	0
order-engine-sdk/src/transaction.rs	0	0
programs/order-engine/src/error.rs	100	100
programs/order-engine/src/instructions/fill.rs	98.71	100
programs/order-engine/src/lib.rs	100	100
server-example/src/config.rs	0	0
server-example/src/main.rs	0	0
server-example/src/server.rs	0	0
webhook-api/src/enums.rs	0	0
webhook-api/src/requests.rs	0	0
webhook-api/src/responses.rs	0	0