added examples for cblas linker options

I realized that I pushed my own LDFLAGS some time ago.
I added examples for possible LDFLAGS and also added a line to the readme
recommending OpenBLAS which as far as I know is the most performant open
BLAS library.
Maybe we could add a script which clones and builds OpenBLAS and then generates cblas.go
with the correct LDFLAGS?