Add some detail on how libraries work.

vhdlpp/README.txt

@@ -28,4 +28,25 @@ LIBRARY FORMAT:
 The vhdlpp program stores libraries as directory that contain
 packages. The name of the directory (in lower case) is the name of the
 library as used on the "import" statement. Within that library, there
-are packages in files named <foo>.pkg.
+are packages in files named <foo>.pkg. For example:
+
+    <directory>/...
+       sample/...
+         test1.pkg
+	 test2.pkg
+       bar/...
+         test3.pkg
+
+Use the "+vhdl-libdir+<directory>" record in a config file to tell
+Icarus Verilog that <directory> is a place to look for libraries. Then
+in your VHDL code, access packages like this:
+
+    library sample;
+    library bar;
+    use sample.test1.all;
+    use bar.test3.all;
+
+The *.pkg files are just VHDL code containing only the package with
+the same name. When Icarus Verilog encounters the "use <lib>.<name>.*;" 
+statement, it looks for the <name>.pkg file in the <lib> library and
+parses that file to get the package header declared therin.