(doc) Exclude GNUstep packages from EPEL

Documentation/SOGoInstallationGuide.asciidoc

@@ -238,6 +238,14 @@ sources, install the following package:
 rpm -ivh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
 ----
 
+SOGo relies on the GNUstep packages provided by Inverse and must not used the
+packages from EPEL. Adjust the repository definition to exclude those packages:
+
+----
+sed -i '/enabled=1/a \
+exclude=gnustep*' /etc/yum.repos.d/epel.repo
+----
+
 For more information on EPEL, visit http://fedoraproject.org/wiki/EPEL/.
 
 Software Installation