github: add sample-lnd.conf to PR checklist

.github/pull_request_template.md

@@ -13,6 +13,7 @@
 - [ ] Code has been formatted with `go fmt`
 - [ ] Protobuf files (`lnrpc/**/*.proto`) have been formatted with
   `make rpc-format` and compiled with `make rpc`
+- [ ] New configuration flags have been added to `sample-lnd.conf`
 - [ ] For code and documentation: lines are wrapped at 80 characters
   (the tab character should be counted as 8 characters, not 4, as some IDEs do
   per default)