document ZSTD_USE_C90_QSORT

diff --git a/lib/README.md b/lib/README.md
index b37f5fc4f3ffeaae05b5b157f44f92a5c4adfe82..9df516c5a1921cb4596a8babe28e44e512e0091b 100644 (file)
--- a/lib/README.md
+++ b/lib/README.md
@@ -193,6 +193,10 @@ The file structure is designed to make this selection manually achievable for an
   and assembly decoding loops. You may want to use this macro if these loops are
   slower on your platform.
 
+- The macro `ZSTD_USE_C90_QSORT` forces usage of C90's `qsort()`,
+  for situations where the code cannot determine that `qsort_r()` is not supported,
+  such as, for example, older versions of `musl`.
+
 #### Windows : using MinGW+MSYS to create DLL
 
 DLL can be created using MinGW+MSYS with the `make libzstd` command.