nixos/btrbk: add release notes

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

@@ -25,6 +25,15 @@
   <section xml:id="sec-release-21.11-new-services">
     <title>New Services</title>
     <itemizedlist>
+      <listitem>
+        <para>
+          <link xlink:href="https://digint.ch/btrbk/index.html">btrbk</link>,
+          a backup tool for btrfs subvolumes, taking advantage of btrfs
+          specific capabilities to create atomic snapshots and transfer
+          them incrementally to your backup locations. Available as
+          <link xlink:href="options.html#opt-services.brtbk.instances">services.btrbk</link>.
+        </para>
+      </listitem>
       <listitem>
         <para>
           <link xlink:href="https://github.com/maxmind/geoipupdate">geoipupdate</link>,

nixos/doc/manual/release-notes/rl-2111.section.md

@@ -10,6 +10,8 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 ## New Services {#sec-release-21.11-new-services}
 
+- [btrbk](https://digint.ch/btrbk/index.html), a backup tool for btrfs subvolumes, taking advantage of btrfs specific capabilities to create atomic snapshots and transfer them incrementally to your backup locations. Available as [services.btrbk](options.html#opt-services.brtbk.instances).
+
 - [geoipupdate](https://github.com/maxmind/geoipupdate), a GeoIP database updater from MaxMind. Available as [services.geoipupdate](options.html#opt-services.geoipupdate.enable).
 
 - [sourcehut](https://sr.ht), a collection of tools useful for software development. Available as [services.sourcehut](options.html#opt-services.sourcehut.enable).