Update 'README.md'

README.md

@@ -50,6 +50,7 @@ ___
 - [Carathéodory's theorem (convex hull)](https://en.wikipedia.org/wiki/Carath%C3%A9odory%27s_theorem_(convex_hull))
 - [Clydesdale](https://en.wikipedia.org/wiki/Clydesdale_horse)
 - [Metric Prefixes](https://en.wikipedia.org/wiki/Metric_prefix)
+- [Offa's Dyke](https://en.wikipedia.org/wiki/Offa%27s_Dyke)
 
  
 ## [Mathematics and Theoretical Computer Science](mathematics.md)