added some links and hints where to get dependency libraries

doc/RTD/source/GettingStarted/compiling_code.rst

@@ -37,10 +37,13 @@ using a build of parallel-HDF5, as SWIFT can leverage this when writing and
 reading snapshots. We recommend using HDF5 > 1.10.x as this is *vastly superior*
 in parallel.
 
+HDF5 is widely available through system package managers.
+
 MPI
 ~~~
 A recent implementation of MPI, such as Open MPI (v2.x or higher), is required,
 or any library that implements at least the MPI 3 standard.
+MPI implementations are widely available through system package managers.
 
 Running SWIFT on OmniPath atchitechtures with Open MPI
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -53,19 +56,25 @@ with ``--mca btl vader,self -mca mtl psm``.
 
 Libtool
 ~~~~~~~
-The build system depends on libtool.
+The build system depends on libtool. Libtool is widely available through system 
+package managers.
 
 FFTW
 ~~~~
-Version 3.3.x or higher is required for periodic gravity.
+Version 3.3.x or higher is required for periodic gravity. FFTW  is widely available
+through system package managers or on http://fftw.org/.
 
 ParMETIS or METIS
 ~~~~~~~~~~~~~~~~~
-One is required for domain decomposition and load balancing.
+One of these libraries is required for domain decomposition and load balancing. 
+Source codes for them libraries are available 
+`here for METIS <https://github.com/KarypisLab/METIS>`_ and 
+`here for ParMETIS <https://github.com/KarypisLab/ParMETIS>`_ .
 
 GSL
 ~~~
-The GSL is required for cosmological integration.
+The GSL is required for cosmological integration. GSL is widely available through
+system package managers.
 
 
 Optional Dependencies
@@ -87,17 +96,23 @@ You can build documentation for SWIFT with DOXYGEN.
 
 Python
 ~~~~~~
-To run the examples, you will need python 3 and some of the standard scientific libraries (numpy, matplotlib).
-Some examples make use of the `swiftsimio <https://swiftsimio.readthedocs.io/en/latest/>`_ library.
+To run the examples, you will need python 3 and some of the standard scientific 
+libraries (numpy, matplotlib). Some examples make use of the 
+`swiftsimio <https://swiftsimio.readthedocs.io/en/latest/>`_ library.
 
 GRACKLE
 ~~~~~~~
-GRACKLE cooling is implemented in SWIFT. If you wish to take advantage of it, you will need it installed.
+GRACKLE cooling is implemented in SWIFT. If you wish to take advantage of it, you 
+will need it installed. It can be found `here <https://github.com/grackle-project/grackle>`_.
+
 
 HEALPix C library
 ~~~~~~~~~~~~~~~~~~~
 
-This is required for making light cone HEALPix maps. Note that by default HEALPix builds a static library which cannot be used to build the SWIFT shared library. Either HEALPix must be built as a shared library or -fPIC must be added to the C compiler flags when HEALPix is being configured.
+This is required for making light cone HEALPix maps. Note that by default HEALPix 
+builds a static library which cannot be used to build the SWIFT shared library. 
+Either HEALPix must be built as a shared library or -fPIC must be added to the C 
+compiler flags when HEALPix is being configured.
 
 CFITSIO
 ~~~~~~~