f add rationale for default value

Author: TheBlueMatt

lightning/src/util/config.rs

@@ -151,6 +151,10 @@ pub struct ChannelConfig {
 	/// This may be allowed to change at runtime in a later update, however doing so must result in
 	/// update messages sent to notify all nodes of our updated relay fee.
 	///
+	/// The default value of a single satoshi roughly matches the market rate on many routing nodes
+	/// as of July 2021. Ajusting it upwards or downwards may change whether nodes route through
+	/// this node.
+	///
 	/// Default value: 1000.
 	pub fee_base_msat: u32,
 	/// The difference in the CLTV value between incoming HTLCs and an outbound HTLC forwarded over