doc: Explain .xdg-app-builder

This directory can grow quite large (my local example here is 900M),
so we should document that it is safe to remove.

doc/xdg-app-builder.xml

@@ -671,6 +671,23 @@
           </variablelist>
     </refsect1>
 
+    <refsect1>
+        <title>Caching</title>
+
+        <para>
+            xdg-app-builder caches sources and partial build results in
+            the .xdg-app-builder subdirectory of the current directory. If you
+            use <option>--keep-build-dirs</option>, build directories for each
+            module are also stored here.
+        </para>
+
+        <para>
+            It is safe to remove the contents of the .xdg-app-builder
+            directory. This will force a full build the next time you build.
+        </para>
+
+    </refsect1>
+
     <refsect1>
         <title>Examples</title>