Adding info to start page and example to build page to match JOSS request

Author: ilyamandel

docs/online-docs/index.rst

@@ -11,6 +11,10 @@ COMPAS
 ======
 
 **C**\ ompact **O**\ bject **M**\ ergers: **P**\ opulation **A**\ strophysics and **S**\ tatistics
+is a publicly available rapid stellar / binary population synthesis code designed so that evolution prescriptions and model parameters are 
+easily adjustable. COMPAS draws properties for a binary star system from a set of initial distributions, and evolves it from 
+zero-age main sequence to the end of its life as two compact remnants. It has been used for inference from observations of 
+gravitational-wave mergers, Galactic neutron stars, X-ray binaries, and luminous red novae.
 
 `... by` `TeamCOMPAS <https://compas.science/index.html>`_
 

docs/online-docs/pages/Getting started/building-COMPAS.rst

@@ -54,6 +54,11 @@ in which all source files are compiled, type::
 The `clean` option instructs ``make`` to remove all existing object files (.o), and the COMPAS executable.  A subsequent ``make`` is then forced to compile
 all source files and link the resultant object files (and external libraries) into a new executable.
 
+The executable can be tested with, e.g.,
+
+    ./COMPAS -v
+
+which will display the code version.
 
 See :doc:`../Developer guide/Developer build/COMPAS-local-build` for a detailed description of ``Makefile`` functionality.