Help: Add release notes for topic 'FindPkgConfig-PKG_CONFIG'

Help/release/dev/FindPkgConfig-PKG_CONFIG.rst

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+FindPkgConfig-PKG_CONFIG
+------------------------
+
+* The :module:`FindPkgConfig` module learned to use the ``PKG_CONFIG``
+  environment variable value as the ``pkg-config`` executable, if set.