debian/README.source: Document how the list of devices in debian/libfprint0.postinst is generated

debian/README.source

@@ -0,0 +1,19 @@
+The libfprint0 postinst script calls udevadmin trigger for all the supported
+USB readers. The list of readers used in that file is generated by calling the
+following awk script:
+
+  /plugdev/ {
+          printf ("\tudevadm trigger --action=add " )
+          for (i=1;i<=NF; i++){
+                  if (match($i,/idVendor/)>0) {
+                          match($i, /"[^"]*"/); printf ("--attr-match=idVendor=%s ",substr($i,RSTART+1,RLENGTH-2))
+                  }
+                  if (match($i,/idProduct/)>0) {
+                          match($i, /"[^"]*"/); printf ("--attr-match=idProduct=%s",substr($i,RSTART+1,RLENGTH-2))
+                  }
+          };
+          printf("\n")
+  }
+
+When preparing a new upstream release, this script should be called against the
+udev .rules file generated during the build of the package