show how to install with Spack; closes #120 (#122)

Author: bast

README.md

@@ -21,3 +21,5 @@ XCFun is licensed under version 2.0 of the Mozilla Public License
 ## Documentation
 
 The documentation is available at https://xcfun.readthedocs.io.
+
+XCFun can now be installed in a [Spack](https://spack.io) environment: https://xcfun.readthedocs.io/using.html#installing-using-spack

docs/using.rst

@@ -20,6 +20,18 @@ To use the library, you will need to:
 
    use xcfun
 
+
+Installing using Spack
+----------------------
+
+XCFun can be installed in a `Spack
+<https://www.spack.io/>`_ environment with::
+
+  spack env create myenv
+  spack env activate myenv
+  spack install xcfun
+
+
 .. _integration:
 
 Integration with your build system