Improve CentOS 6 information.

Author: enolfc

doc/source/installation.rst

@@ -38,15 +38,41 @@ Or, if you have virtualenvwrapper installed::
 
     $ mkvirtualenv caso
     $ pip install caso
+
+CentOS 6
+********
 
-On CentOS 6, you can use Software Collections to install Python 2.7 and then libffi-devel, which is also required::
+On CentOS 6, you can use Software Collections to install Python 2.7::
     
     $ yum -y install centos-release-SCL
     $ yum -y install python27
-    $ yum -y install libffi-devel
+
+There are also some dependencies of the packages used by ``cASO`` that need to
+be installed (gcc, libffi-devel adn openssl-devel)::
+
+    $ yum -y install gcc libffi-devel openssl-devel
     
-You can then install pip for that version of Python and use that to install ``cASO``::
+You can then install pip for that version of Python and use that to install
+``cASO``::
 
-    $ scl enable python27
+    $ scl enable python27 bash
     $ easy_install-2.7 pip
     $ pip install caso
+    $ exit    # this terminates bash with the SCL python2.7
+
+In this case you can later on use ``caso-extract`` with the following command
+line::
+
+    $ scl enable python27 caso-extract
+
+Alternatively, if you want to use a virtualenv::
+
+    $ scl enable python27 bash
+    $ virtualenv caso
+    $ . caso/bin/activate
+    $ pip install caso
+    $ exit    # this terminates bash with the SCL python2.7
+
+Running from the virtualenv::
+
+    $ scl enable python27 caso/bin/caso-extract