adding Ignition config to the glossary

Author: kalexand-rh

contributing_to_docs/term_glossary.adoc

@@ -204,6 +204,15 @@ Both the username and list of groups the user belongs to.
 
 Usage: image(s)
 
+''''
+=== Ignition config
+
+Usage: Ignition config file or Ignition config files
+
+The file that Ignition uses to configure Red Hat Enterprise Linux CoreOS during
+operating system initialization. The installation program generates different
+Ignition config files to initialize bootstrap, master, and worker nodes.
+
 == K
 
 ''''