docs: Add documentation of --experimental-dual-fund option

doc/lightningd-config.5

@@ -592,6 +592,16 @@ remote node has opened a channel but claims it used the incorrect txid
 (and the channel hasn't been used yet at all) this allows them to
 negotiate a clean shutdown with the txid they offer\.
 
+
+ \fBexperimental-dual-fund\fR
+
+
+Specifying this enables support for the dual funding protocol,
+allowing both parties to contribute funds to a channel\. The decision
+about whether to add funds or not to a proposed channel is handled
+automatically by a plugin that implements the appropriate logic for
+your needs\. The default behavior is to not contribute funds\.
+
 .SH BUGS
 
 You should report bugs on our github issues page, and maybe submit a fix
@@ -617,4 +627,4 @@ Main web site: \fIhttps://github.com/ElementsProject/lightning\fR
 Note: the modules in the ccan/ directory have their own licenses, but
 the rest of the code is covered by the BSD-style MIT license\.
 
-\" SHA256STAMP:b0cdc467650b05758f90f2523ab8b45e526ac818e426cb2c2bc42cb98673e373
+\" SHA256STAMP:cdddb53037da4e67505114769586ed56914827c584a073956387933bc09e7aa9

doc/lightningd-config.5.md

@@ -490,6 +490,14 @@ remote node has opened a channel but claims it used the incorrect txid
 (and the channel hasn't been used yet at all) this allows them to
 negotiate a clean shutdown with the txid they offer.
 
+ **experimental-dual-fund**
+ 
+Specifying this enables support for the dual funding protocol,
+allowing both parties to contribute funds to a channel. The decision
+about whether to add funds or not to a proposed channel is handled
+automatically by a plugin that implements the appropriate logic for
+your needs. The default behavior is to not contribute funds.
+
 BUGS
 ----