Added an introductory blurb about GC that I had lying around.

doc/ref/scheme-memory.texi

@@ -2,6 +2,21 @@
 @node Memory Management
 @chapter Memory Management and Garbage Collection
 
+Guile uses a @emph{garbage collector} to manage most of its objects.
+This means that the memory used to store a Scheme string, say, is
+automatically reclaimed when no one is using this string any longer.
+This can work because Guile knows enough about its objects at run-time
+to be able to trace all references between them.  Thus, it can find
+all 'life' objects (objects that are still in use) by starting from a
+known set of 'root' objects and following the links that these objects
+have to other objects, and so on.  The objects that are not reached by
+this recursive process can be considered 'dead' and their memory can
+be used for new objects.
+
+When you are programming in Scheme, you don't need to worry about the
+garbage collector.  When programming in C, there are a few rules that
+you must follow so that the garbage collector can do its job.
+
 @menu
 * Garbage Collection::
 * Memory Blocks::