Explaining the origin of the arb numerics code.

arb/README.md

@@ -0,0 +1,15 @@
+# Arb numerics
+
+These are some initial work by Brad from 2020/2021, where he was extending the
+'special functions' approach of Devreese, in order to try and evaluate
+frequency dependent mobility functions, at the level of the counter integration
+in FHIP. 
+
+Shortly after all this work, he realised that actually brute-forcing the
+intengral with QuadGK *BEFORE* making the shift to a contour integration had
+far better numerics, and was faster and had simpler code than using these
+special functions (because we had to take them to ridiculous accuracy). 
+
+These special functions supplement the Julia ArbNumerics package, but we never
+pushed the code upstream. :(
+