math: add readme about ulp check tool

diff --git a/src/math/gen/README b/src/math/gen/README
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+++ b/src/math/gen/README
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+tools for generating testcases and checking ulp error of math functions
+
+gen: math functions implemented with mpfr
+mgen: math functions from libm
+check: compare input to libm and report errors
+
+# check asinh in the [0.125,0.5] domain over 100k points and report >1.5ulp errors
+./rnd -a 0x1p-3 -b 0x1p-1 -n 100000 |./gen asinh |./check asinh 1.5