Create readme.md

Author: winner10920

libraries/SDU/extras/SDUBoot/readme.md

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+to add new board
+change ss pin to desired pin for new board
+export compiled binary
+run (cat "SDU.bin" | xxd -i > nano_33_iot.h) file names are example
+move new hex file to src/boot, and modify SDU.cpp with a new if else statement
+should compile now and notnget unsupported board issue