README: Add link to https://www.unidata.ucar.edu/software/udunits/

Author: df7cb

README.md

@@ -543,6 +543,7 @@ References
 * https://en.wikipedia.org/wiki/United_States_customary_units
 * GNU Units: https://www.gnu.org/software/units/
 * Similar library for Ada: http://www.dmitry-kazakov.de/ada/units.htm
+* Similar library for C: https://www.unidata.ucar.edu/software/udunits/
 
 License
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