docs: try and clarify that you should look in other directories

Author: karlp

examples/stm32/f1/README.md

@@ -0,0 +1,5 @@
+Examples in these directories are not _only_ for the board listed, they are
+simply preconfigured for those boards.  If you do not find an example of the
+peripheral you want to use listed for your board, please look at the other
+boards.  The differences between boards for the examples is normally only
+LEDs and buttons on different pins, and different memory sizes.