eeshow/README: discuss use of *-cache.lib; TODO: areas enclose closed paths

eeshow/README

@@ -46,7 +46,7 @@ many limitations, especially in the following areas:
 
 - interactive use is not streamlined yet, e.g., revision selection
 
-- there may be bugs
+- there may be unknown bugs
 
 More details on pending items can be found in the file TODO.
 
@@ -94,6 +94,18 @@ overridden. E.g., the above example selects also revision 6a9f71 of
 neo900.lib.
 
 
+Using the component library cache
+---------------------------------
+
+Eeschema copies all components it uses to a file named <project>-cache.lib
+When operating on files, this library can be used as a convenient
+alternative to listing all the libraries that contributed content.
+
+However, when using eeshow with a revision history, the library cache
+should only be used if it is also kept in the repository. Otherwise,
+library changes in older revisions may get unnoticed.
+
+
 Non-interactive differences
 ===========================