PkgC: Add release note

This was missed in commit 8555c33d92 (PkgC: Add cmake_pkg_config(EXTRACT)
command, 2024-07-22).

Help/release/dev/cmake-pkg-config.rst

@@ -0,0 +1,8 @@
+cmake-pkg-config
+----------------
+
+* The :command:`cmake_pkg_config` command was added as an endpoint for using
+  CMake's native pkg-config format parser. The only supported option in this
+  release is ``EXTRACT``, which provides low-level access to the values
+  produced by parsing a pkg-config file. For most users, this is not yet a
+  suitable replacement for the :module:`FindPkgConfig` module.