fix(doc): additional config for CLN to eclair

Without these configs CLN keysend payments to eclair will fail.
This is because CLN uses 22 final expiry delta which is less than
eclair's default (30).

Author: Abdulkbk

README.md

@@ -37,6 +37,13 @@ of the simulator uses keysend to execute payments, which must be enabled as foll
 * CLN: enabled by default
 * Eclair: `-Declair.features.keysend=optional` (or `--features.keysend=optional` if you're using Polar)
 
+NOTE: for CLN `keysend` to work with eclair, you need to add additional config to eclair:
+```
+-Declair.channel.min-final-expiry-delta-blocks=N
+-Declair.channel.fulfill-safety-before-timeout-blocks=M
+```
+where N and M are numbers, and N must be larger than M (N must be 22, CLN's default, or more)
+
 ## Getting Started
 
 Once you have all the pre-requisites installed, clone the repo: