docs: add chapter on 3rd-party extensions

Fixes #3459

docs/3rd-party.adoc

@@ -0,0 +1,18 @@
+[[ch-3rd-party]]
+== Third-Party Tools and Extensions
+
+Here is a collection of tools and extensions that relate to Home
+Manager. Note, these are maintained outside the regular Home Manager
+flow so quality and support may vary wildly. If you encounter problems
+then please raise them in the corresponding project, not as issues in
+the Home Manager tracker.
+
+If you have made something interesting related to Home Manager then
+you are encouraged to create a PR that expands this chapter.
+
+[[sec-3rd-party-module-collections]]
+=== Module Collections
+
+- https://github.com/schuelermine/xhmm[xhmm — extra Home Manager modules]
++
+A collection of modules maintained by Anselm Schüler.

docs/manual.xml

@@ -35,6 +35,7 @@
  <xi:include href="nix-flakes.xml" />
  <xi:include href="writing-modules.xml" />
  <xi:include href="contributing.xml" />
+ <xi:include href="3rd-party.xml" />
  <xi:include href="faq.xml" />
  <appendix xml:id="ch-options">
   <title>Configuration Options</title>