Add license for FPGA ring oscillator test

GPIO/iCE40/ring.v

-// Cause yosys to throw an error when we implicitly declare nets
+//
+// ring.v
+//
+// iCE40 FPGA ring oscillator test
+//
+// Erik Strand 9/30/21
+//
+// This work may be reproduced, modified, distributed,
+// performed, and displayed for any purpose, but must
+// acknowledge this project. Copyright is retained and
+// must be preserved. The work is provided as is; no
+// warranty is provided, and users accept all liability.
+//
+
+// Make yosys throw an error when we implicitly declare nets.
 `default_nettype none
 
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