Merge branch 'grackle_odc' into 'master'

Add doc for grackle cooling

See merge request !649

doc/RTD/source/Cooling/index.rst

@@ -48,6 +48,16 @@ supposes an equilibrium and computes the fractions automatically.
 In order to compile SWIFT with Grackle, you need to provide the options ``with-grackle``
 and ``with-chemistry``.
 
+You will need a Grackle version later than 3.1. To compile it, run
+the following commands from the root directory of Grackle:
+``./configure; cd src/clib``.
+Update the variables ``LOCAL_HDF5_INSTALL`` and ``MACH_INSTALL_PREFIX`` in
+the file ``src/clib/Make.mach.linux-gnu``.
+Finish with ``make machine-linux-gnu; make && make install``.
+If you encounter any problem, you can look at the `Grackle documentation <https://grackle.readthedocs.io/en/latest/>`_
+
+You can now provide the path given for ``MACH_INSTALL_PREFIX`` to ``with-grackle``.
+
 Eagle
 ~~~~~