update README: Add debugging hints

Changelog

@@ -1,28 +1,25 @@
 Version 2.11
 ---------
+ * Update README
  * Raise NBD connection error if `--tls` option is set but installed libnbd
  bindings for python do not support required features.
  * Set TLS connection options to TLS_REQUIRE instead of TLS_ALLOW which would
  fallback to non-encrypted data transfer.
 
-
 Version 2.10
 ---------
  * Add --ssh-port option.
 
-
 Version 2.9
 ---------
  * Fix: backup with compression enabled fails: unsupported operand type(s) for
  +: 'int' and 'dict' (#177)
 
-
 Version 2.8
 ---------
  * Add packages compatible to fedora 39 to package build (#174)
  * Show total saved disk size in human readable output (#173)
 
-
 Version 2.7
 ---------
  * Update README
@@ -31,7 +28,6 @@ Version 2.7
  * Detect remote connection based on libvirt URI, checking hostnames could lead
  to situation where local backup is detected as remote backup (#170)
 
-
 Version 2.6
 ---------
  * Fix IndexError exception if auth file is used in qemu uri (#167)
@@ -44,7 +40,6 @@ Version 2.6
  * Update README regards OVIRT/RHEV/OLVM (no mechanism available: No worthy
  mechs found)
 
-
 Version 2.5
 ---------
  * Move some log messages from info to debug loglevel
@@ -59,7 +54,6 @@ Version 2.5
  * Do not attempt to freeze filesystems if virtual machine is in paused state
  (#166)
 
-
 Version 2.4
 ---------
  * Update RADME

README.md

@@ -61,6 +61,7 @@ of your `kvm/qemu` virtual machines.
   - [OpenNebula](#opennebula)
 - [Authentication](#authentication)
 - [Backup I/O and performance: scratch files](#backup-io-and-performance-scratch-files)
+- [Debugging](#debugging)
 - [FAQ](#faq)
   - [The thin provisioned backups are bigger than the original qcow images](#the-thin-provisioned-backups-are-bigger-than-the-original-qcow-images)
   - [Backup fails with "Cannot store dirty bitmaps in qcow2 v2 files"](#backup-fails-with-cannot-store-dirty-bitmaps-in-qcow2-v2-files)
@@ -983,6 +984,16 @@ same I/O performance requirements as the backup target.
 The free space on the default scratch directory (`/var/tmp`) must be enough to
 be able to keep all fleecing data while the backup is active.
 
+# Debugging
+
+To get more detailed debug output use `--verbose` option. To enable NBD
+specific debugging output export LIBNBD_DEBUG environment variable prior to
+executing the backup or restore:
+
+```
+export LIBNBD_DEBUG=1
+```
+
 # FAQ
 ## The thin provisioned backups are bigger than the original qcow images