Update README_ACC.md (Xeon phi KNL)

specfem3d/README_ACC.md

@@ -86,6 +86,8 @@ FCFLAGS = -g -O3 -qopenmp -xMIC-AVX512 -DUSE_FP32 -DOPT_STREAMS -align array64by
 FCFLAGS_f90 = -mod ./obj -I./obj -I.  -I. -I${SETUP} -xMIC-AVX512
 CPPFLAGS = -I${SETUP}  -DFORCE_VECTORIZATION  -xMIC-AVX512
 ```
+Note: Be careful, in most machines login node does not have the same instruction set so, in order to compile with the right instruction set, you'll have to compile on a compute node (salloc + ssh)
+
 
 Finally compile with make:
 ```shell