docs: Add starter doc on debugging lnd.

docs/debugging_lnd.md

@@ -0,0 +1,47 @@
+# Table of Contents
+1. [Overview](#overview)
+1. [Debug Logging](#debug-logging)
+1. [Capturing pprof data with `lnd`](#capturing-pprof-data-with-lnd)
+
+## Overview
+
+`lnd` ships with a few useful features for debugging, such as a built-in
+profiler and tunable logging levels. If you need to submit a bug report
+for `lnd`, it may be helpful to capture debug logging and performance
+data ahead of time.
+
+## Debug Logging
+
+You can enable debug logging in `lnd` by passing the `--debuglevel` flag. For
+example, to increase the log level from `info` to `debug`:
+
+```
+$ lnd --debuglevel=debug
+```
+
+You may also specify logging per-subsystem, like this:
+
+```
+$ lnd --debuglevel=<subsystem>=<level>,<subsystem2>=<level>,...
+```
+
+## Capturing pprof data with `lnd`
+
+`lnd` has a built-in feature which allows you to capture profiling data at
+runtime using [pprof](https://golang.org/pkg/runtime/pprof/), a profiler for
+Go. The profiler has negligible performance overhead during normal operations
+(unless you have explictly enabled CPU profiling).
+
+To enable this ability, start `lnd` with the `--profile` option using a free port.
+
+```
+$ lnd --profile=9736
+```
+
+Now, with `lnd` running, you can use the pprof endpoint on port 9736 to collect
+runtime profiling data. You can fetch this data using `curl` like so:
+
+```
+$ curl http://localhost:9736/debug/pprof/goroutine?debug=1
+...
+```