Add keysend to lnd via docker-fragment (#390)

* Create  opt-lnd-keysend.yml 

Set lightning config keysend parameter.

* Added keysend

Added description for docker-fragments/opt-lnd-keysend.yml

README.md

@@ -153,6 +153,7 @@ Available `BTCPAYGEN_ADDITIONAL_FRAGMENTS` currently are:
 * [opt-save-storage-xs](docker-compose-generator/docker-fragments/opt-save-storage-xs.yml) will keep around 3 months of blocks (prune BTC for 25 GB)
 * [opt-save-storage-xxs](docker-compose-generator/docker-fragments/opt-save-storage-xxs.yml) will keep around 2 weeks of blocks (prune BTC for 5 GB) (lightning not supported)
 * [opt-lnd-autopilot](docker-compose-generator/docker-fragments/opt-lnd-autopilot.yml) will activate auto pilot on LND. (5 channels, 60% of allocation)
+* [opt-lnd-keysend](docker-compose-generator/docker-fragments/opt-lnd-keysend.yml) will activate keysend on LND.
 * [opt-save-memory](docker-compose-generator/docker-fragments/opt-save-memory.yml) will decrease the default dbcache at the expense of longer synchronization time. (Useful if your machine is less than 2GB)
 * [opt-more-memory](docker-compose-generator/docker-fragments/opt-more-memory.yml) will increase the default dbcache to make synchronization faster (Useful if your machine is has around 4GB)
 * [opt-add-btcqbo](docker-compose-generator/docker-fragments/opt-add-btcqbo.yml) will allow you to create an invoice on Quickbooks which include a way for your customer to pay on BTCPay Server (More information on this [github repository](https://github.com/JeffVandrewJr/btcqbo/), this add-on is maintained by [JeffVandrewJr](https://github.com/JeffVandrewJr), see more on [this video](https://www.youtube.com/watch?v=srgwL9ozg6c))

docker-compose-generator/docker-fragments/opt-lnd-keysend.yml

@@ -0,0 +1,7 @@
+version: "3"
+
+services:
+  lnd_bitcoin:
+    environment:
+      LND_EXTRA_ARGS: |
+        accept-keysend=1