docs: Add instructions for paying an Eclair offer

docs/test_pay_offer.md

@@ -0,0 +1,70 @@
+## Eclair testing instructions
+
+These instructions assume you already have bitcoind and lnd nodes set up on regtest.
+
+The network we'll build will look like this:
+
+`LND --> Eclair --> Eclair2`
+
+Eclair2 will create an offer and we'll make sure lnd can pay the offer with the help of lndk. 
+
+For Eclair to create an offer to pay, we need to install the tipjar plugin. Pull down this branch (https://github.com/ACINQ/eclair-plugins/pull/6/files) and build `eclair-plugins`:
+
+`mvn package -DskipTests`
+
+The resulting jar will be located at `eclair-plugins/bolt12-tip-jar/target/bolt12-tip-jar-0.9.1-SNAPSHOT.jar`
+
+Set up two eclair directories, for example at `~/eclair1` and `~/eclair2` with the following configuration within (at `eclair.conf`):
+
+```
+eclair {
+  chain=regtest
+
+  server {
+    port=9995
+    public-ips = [ "https://localhost" ]
+  }
+
+  bitcoind {
+    rpcuser="PASSWORD"
+    rpcpassword="PASSWORD"
+    rpcport=18443
+    zmqblock="tcp://127.0.0.1:28334"
+    zmqtx="tcp://127.0.0.1:29335"
+  }
+
+  api {
+    enabled = true
+    port = 8181
+    password = "testing2"
+  }
+
+  features {
+    option_route_blinding = optional
+  }
+
+  tip-jar {
+    description = "donation to eclair"
+    default-amount-msat = 100000000 // Amount to use if the invoice request does not specify an amount
+    max-final-expiry-delta = 1000 // How long (in blocks) the route to pay the invoice will be valid
+  }
+}
+```
+
+Just:
+- Set the correct bitcoind values.
+- Make sure that `server.port` and `api.port` are different for each Eclair node. 
+
+Install the dependencies for Eclair and install Eclair with `mvn package -DskipTests`. After unzipping the eclair bin (located in `target`), run two eclair nodes, each pointing to the appropriate datadirs we set above, and load in the tipjar plugin:
+
+`./eclair-node.sh /eclair-plugins/bolt12-tip-jar/target/bolt12-tip-jar-0.9.1-SNAPSHOT.jar -Declair.datadir="~/eclair1`
+
+`./eclair-node.sh ../../../../../eclair-plugins/bolt12-tip-jar/target/bolt12-tip-jar-0.9.1-SNAPSHOT.jar -Declair.datadir="~/eclair2`
+
+Using [Eclair's API](https://acinq.github.io/eclair), open two channels: 1) from LND to Eclair1, 2) from Eclair1 to Eclair2. Mine several blocks using bitcoind and make sure those channels are confirmed.
+
+Have Eclair2 create an offer with:
+
+`eclair-cli tipjarshowoffer`
+
+Then pay the offer. Run `lndk-cli pay-offer` using the instructions above.