added sendpsbt documentation

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

doc/index.rst

@@ -69,6 +69,7 @@ c-lightning Documentation
    lightning-sendpsbt <lightning-sendpsbt.7.md>
    lightning-setchannelfee <lightning-setchannelfee.7.md>
    lightning-signmessage <lightning-signmessage.7.md>
+   lightning-signpsbt <lightning-signpsbt.7.md>
    lightning-txdiscard <lightning-txdiscard.7.md>
    lightning-txprepare <lightning-txprepare.7.md>
    lightning-txsend <lightning-txsend.7.md>

doc/lightning-signpsbt.7.md

@@ -0,0 +1,61 @@
+lightning-signpsbt -- Command to sign a wallet's inputs on a provided bitcoin transaction (PSBT).
+============================================================
+
+SYNOPSIS
+--------
+
+**signpsbt** \[psbt\]
+
+DESCRIPTION
+-----------
+
+The **signpsbt** is a low-level RPC command which sign a PSBT.
+
+- *psbt*: A string that rappresent the hexadecimal of the psbt.
+
+EXAMPLE JSON REQUEST
+------------
+```json
+{
+  "id": 82,
+  "method": "signpsbt",
+  "params": {
+    "psbt": "some_psbt"
+  }
+}
+```
+
+RETURN VALUE
+------------
+
+On success, a object will be return with a string that rappresent the hexadecimal value of psbt.
+
+- *psbt*: A string that rappresent the hexadecimal dump of the psbt.
+
+On failure, one of the following error codes may be returned:
+
+- -32602. Error in given parameters or there isn't wallet inputs to sign.
+
+EXAMPLE JSON RESPONSE
+-----
+```json
+{
+    "psbt": "some_psbt"
+}
+```
+
+
+AUTHOR
+------
+
+Vincenzo Palazzo <<vincenzo.palazzo@protonmail.com>> wrote the initial version of this man page, but many others did the hard work of actually implementing this rpc command.
+
+SEE ALSO
+--------
+
+lightning-fundpsbt(7), lightning-sendpsbt(7)
+
+RESOURCES
+---------
+
+Main web site: <https://github.com/ElementsProject/lightning>