Improve Getting Started by basic information

general/getting_started.rst

@@ -1,2 +1,29 @@
 Getting started
 ===============
+
+Overview
+--------
+
+OPM is a set of tools working above Nagios (or similar)
+to store into a database the perfdata retrieved by any Nagios agent
+and graph them in a web dashboard.
+
+check_pgactivity (https://github.com/OPMDG/check_pgactivity) is a Nagios agent
+dedicated to PostgreSQL.
+It is technically separated from OPM.
+
+
+Requirements & Installation
+---------------------------
+
+We suppose that you have in place and configured to your need:
+  * a working Nagios (or similar) installation
+  * agents displaying perfdata (e.g. check_pgactivity or other Nagios probes)
+  * an empty PostgreSQL database (9.3 or later) on the instance of your choice
+    to store the data.
+
+See the :doc:`Installation <../opm-core/Installation>` page to install:
+  * the :ref:`opm_core <opm_core>` and :ref:`wh_nagios <wh_nagios>` extensions
+  * the :ref:`nagios dispatcher script<nagios_and_nagios_dispatcher>` to
+    retrieve the perfdata
+  * the :ref:`web user interface <user_interface>`.

opm-core/Installation.rst

@@ -14,6 +14,8 @@ The tool ``pg_config`` is required. Distributions packages put it usually in ``/
 
 We suppose that the repositories **opm-core** (https://github.com/OPMDG/opm-core) and **opm-wh_nagios** (https://github.com/OPMDG/opm-wh_nagios) are stored into **/usr/local/src/opm/** (your OPM directory).
 
+.. _opm_core:
+
 OPM core
 --------
 
@@ -253,6 +255,8 @@ and start the job::
 
     root:~# initctl start nagios_dispatcher
 
+.. _user_interface:
+
 User interface
 --------------