Added pkgconf as prerequisite

Author: rhdtownsend

docs/source/ref-guide/installation.rst

@@ -19,7 +19,8 @@ components:
   interface to LAPACK
 * The :git:`fypp <aradi/fypp>` Fortran pre-processor
 * The `HDF5 <https://www.hdfgroup.org/solutions/hdf5/>`__ data management library
-
+* The `pkgconf <http://pkgconf.org/>`__ package management tool
+  
 On Linux and MacOS platforms, these components are bundled together in
 the `MESA Software Development Kit (SDK) <mesa-sdk_>`__. Using this SDK
 is `strongly` recommended.