added readme regarding quadmath build

Author: jniemann66

x64/minGW-W64/QuadMath-readme.txt

@@ -0,0 +1,9 @@
+ReSampler-QuadMath.exe
+======================
+
+Experimental Quadruple-precision version of ReSampler
+
+* uses Quadruple-Precision (128-bit floating-point) for calculations
+* very slow, because CPU doesn't have 128-bit floating point, so it's implemented in software
+* uses the GNU quad-precision math library
+* Currently no method of reading quad-precision input files. (The best you can do is 64f)