Update README.rst
Update README.rst

Update for better installation instructions, mentioning the required `filer` installation steps
divio/cmsplugin-filer,douwevandermeij/cmsplugin-filer,eliasp/cmsplugin-filer,nephila/cmsplugin-filer,wlanslovenija/cmsplugin-filer,douwevandermeij/cmsplugin-filer,divio/cmsplugin-filer,ImaginaryLandscape/cmsplugin-filer,divio/cmsplugin-filer,sephii/cmsplugin-filer,jschneier/cmsplugin-filer,creimers/cmsplugin-filer,centralniak/cmsplugin-filer,yvess/cmsplugin-filer,NB-Dev/cmsplugin-filer,jschneier/cmsplugin-filer,nephila/cmsplugin-filer,skirsdeda/cmsplugin-filer,alsoicode/cmsplugin-filer,nephila/cmsplugin-filer,wlanslovenija/cmsplugin-filer,stefanfoulis/cmsplugin-filer,brightinteractive/cmsplugin-filer,jrutila/cmsplugin-filer,jschneier/cmsplugin-filer,creimers/cmsplugin-filer,stefanfoulis/cmsplugin-filer,dreipol/cmsplugin-filer,NB-Dev/cmsplugin-filer,skirsdeda/cmsplugin-filer,alsoicode/cmsplugin-filer,sephii/cmsplugin-filer,ImaginaryLandscape/cmsplugin-filer,creimers/cmsplugin-filer,alsoicode/cmsplugin-filer,skirsdeda/cmsplugin-filer,eliasp/cmsplugin-filer,divio/cmsplugin-filer,yvess/cmsplugin-filer,wlanslovenija/cmsplugin-filer,yvess/cmsplugin-filer,yvess/cmsplugin-filer,brightinteractive/cmsplugin-filer,eliasp/cmsplugin-filer,sephii/cmsplugin-filer,stefanfoulis/cmsplugin-filer,dreipol/cmsplugin-filer,stefanfoulis/cmsplugin-filer,douwevandermeij/cmsplugin-filer,brightinteractive/cmsplugin-filer,centralniak/cmsplugin-filer,jrutila/cmsplugin-filer