Add include pattern example for .conanignore (#3879)

reference/commands/config.rst

@@ -54,6 +54,14 @@ are copied over to the user's Conan home folder.
 This file uses `fnmatch <https://docs.python.org/3/library/fnmatch.html>`_ patterns
 to match over the folder contents, excluding those entries that match from the config installation.
 See `conan-io/command-extensions's .conanignore <https://github.com/conan-io/command-extensions/blob/main/.conanignore>`_ for an example of such a file.
+You can force certain files to be copied over by using the ``!`` negation syntax:
+
+.. code-block:: text
+
+    # Ignore all files
+    *
+    # But copy the file named "settings.yml"
+    !settings.yml
 
 
 **Examples**: