Package conventions: Add convention for mechanical pads

modules/library-conventions/pages/packages.adoc

@@ -84,10 +84,20 @@ The following conventions apply to package names:
 
 == Pads
 
-* *Always add all pads of packages*, even those which are not always connected.
-  For example, the package "TO220" has a metal tab, so you should define it as
-  a pad, no matter if it's often not connected (and even not connectable when
-  mounted vertically).
+* *Always add all (electrical) pads of packages*, even those which are not
+  always connected. For example the package "TO220" has a metal tab, so you
+  should define it as a pad, even though it's often not connected (and even
+  not connectable when mounted vertically).
+** If there are multiple pads which have a package-internal connection
+   (e.g. shielding pads of an USB connector), add them as one single package
+   pad and connect all corresponding footprint pads to it to make LibrePCB
+   aware of this internal connection.
+** If there are pads for pure mechanical purpose without any internal
+   connection at all (e.g. split solder tabs of a plastic connector), don't
+   add any package pads for them -- just add footprint pads and leave them
+   unconnected. If in doubt, or if connected to a metal cover of significant
+   size, consider those pads as electrically relevant and tread them like
+   normal pads to allow connecting them in devices and schematics when desired.
 * *Use pad names according {ipc7351-pdf}[IPC-7351]* (if applicable). For
   packages which are not covered by IPC-7351:
 ** If the function of a pad is absolutely clear, choose its abbreviated