Update README.md: add mention of -f unroll loops option for gcc

Author: karpathy

README.md

@@ -144,6 +144,8 @@ The fastest throughput I saw so far on my MacBook Air (M1) so far is with `make
 
 You can also experiment with replacing `gcc` with `clang`.
 
+If compiling with gcc, try experimenting with `-funroll-all-loops`, see PR [#183](https://github.com/karpathy/llama2.c/pull/183)
+
 ### OpenMP
 Big improvements can also be achieved by compiling with OpenMP, which "activates" the `#pragma omp parallel for` inside the matmul and attention, allowing the work in the loops to be split up over multiple processors.
 You'll need to install the OpenMP library and the clang compiler first (e.g. `apt install clang libomp-dev` on ubuntu). I was not able to get improvements from OpenMP on my MacBook, though. Then you can compile with `make runomp`, which does: