fix neon compiler flag in README.md
fix cflags to enable ARM neon.

Was unable to compile with listing provided. Searching https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html indicated need for single-quotes. Adding single quote = successful compile (CHIP single-board-computer, ARM R8 single core ARMv7 + vpf3 instructions).

(note: this compile was tpruvot's cpuminer-multi fork, but can't file an issue on a fork, so maybe this will help someone else. using `cflags -march=native -mcpu=native -mtune=native -mfloat-abi='hard' -mfpu='neon'` gained me ~6kH/s decred compared to just running build.sh.)