Add introductory paragraph

Author: nxadm

doc/Language/containers.pod6

@@ -4,6 +4,8 @@
 
 =SUBTITLE A low-level explanation of Perl 6 containers
 
+This section explains the levels of indirection involved in dealing with variables and container elements. The difference types of containers used in Perl 6 are explained and the actions applicable to them like assigning, binding and flattening. More advanced topics like self-referencial data, type contraints and custom containers are discussed at the end.
+
 =head1 What is a variable?
 
 Some people like to say "everything is an object", but in fact a variable is