README: retire libmaple

As of March 2015, the LeafLabs Maple line and the libmaple library are
end-of-life and no longer supported by LeafLabs. The design files for
Maple and Maple Mini will remain available on GitHub, under a CC-BY-SA
2.0 license, for anyone who wants to recreate or reimagine these boards.

Consider checking out the resources and community at www.stm32duino.com
instead.

Signed-off-by: Perry Hung <[email protected]>
Reviewed-by: Marti Bolivar <[email protected]>

Author: iperry

README

@@ -1,3 +1,9 @@
+[INACTIVE] libmaple
+
+libmaple is inactive and no longer accepting submissions. Please consider
+www.stm32duino.com instead.
+
+------------------------------------------------------------------------------
 
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