Add NixOS package link to installation docs.

NEWS

@@ -1,6 +1,6 @@
 1.7.10.dev0
- * #501: Optionally error if a source directory does not exist when "source_directories_must_exist"
-   option in location configuration section is true.
+ * #501: Optionally error if a source directory does not exist via "source_directories_must_exist"
+   option in borgmatic's location configuration.
  * #618: Support for BORG_FILES_CACHE_TTL environment variable via "borg_files_cache_ttl" option in
    borgmatic's storage configuration.
 

docs/how-to/set-up-backups.md

@@ -94,6 +94,7 @@ installing borgmatic:
  * [openSUSE](https://software.opensuse.org/package/borgmatic)
  * [macOS (via Homebrew)](https://formulae.brew.sh/formula/borgmatic)
  * [macOS (via MacPorts)](https://ports.macports.org/port/borgmatic/)
+ * [NixOS](https://search.nixos.org/packages?show=borgmatic&sort=relevance&type=packages&query=borgmatic)
  * [Ansible role](https://github.com/borgbase/ansible-role-borgbackup)
  * [virtualenv](https://virtualenv.pypa.io/en/stable/)