add Google Compute Engine
Update README.md

Add section for building on Google Compute Engine, at least until OpenBLAS 0.2.9 is released and adopted with the patch for Sandybridge.
Add links to GCE and Julia Ferraioli's super blog.

Thanks @jakebolewski for the pointer to @juliaferraioli
clarify that Google does not own Sandybridge :)
clarify
Merge pull request #6301 from cmundi/gce

Help people get started with Julia on Google Compute Engine