[skip ci]docs: transaction labels for on-chain transactions (#284)

* [skip ci]docs: transaction labels for on-chain transactions

Add information about setting transaction labels
for on-chain transactions related to swaps.
Mention limitation that only peerswap LNDs
have on-chain labels for now.

* [skip ci]docs: transaction labels for lbtc

For LBTC transactions,
elementsd `SetLabel` will attach a label to the associated address.

docs/usage.md

@@ -185,3 +185,22 @@ For CLN:
 `lightning-cli peerswap-allowswaprequests [bool] ## 1 to allow, 0 to disallow`
 For LND:
 `pscli allowswaprequests --allow_swaps=[bool] ## true to allow, false to disallow`
+
+## transaction labels
+To make the related transactions identifiable, peerswap sets the label.
+The label of the on-chain transaction corresponding to the swap is set as follows.
+
+* peerswap -- Opening(swap id=b171ee)
+* peerswap -- ClaimByCoop(swap id=b171ee)
+* peerswap -- ClaimByCsv(swap id=b171ee)
+* peerswap -- ClaimByInvoice(swap id=b171ee)
+
+The way the label is set up in each wallet is different.
+
+For CLN:
+Currently, it is not possible to set a label on the cln wallet
+
+For LND:
+To check the label attached to a transaction use `lncli listchaintxns`.
+
+For LBTC transactions, elementsd `SetLabel` will attach a label to the associated address.