Docs: Multi-threading in BLAS

CONTRIBUTING.md

@@ -63,6 +63,15 @@ cmake -D CMAKE_BUILD_TYPE=Release \
 cmake --build build_release --config Release
 ```
 
+When benchmarking, make sure to disable multi-threading in the BLAS library, as it may interfere with the results:
+
+```sh
+export OPENBLAS_NUM_THREADS=1 # for OpenBLAS
+export MKL_NUM_THREADS=1 # for Intel MKL
+export VECLIB_MAXIMUM_THREADS=1 # for Apple Accelerate
+export BLIS_NUM_THREADS=1 # for BLIS
+```
+
 ## Python
 
 Testing: