Speeding up builds guide

A guide about speeding up tests. Parallelizing tests across multiple VMs is taken from @roidrage's post, the rest is for now a short note about the fact that we have 1.5 cores per VM, which may allow to parallelize on one machine.

I would also write something about caching the dependencies, but maybe it should wait till we have an automated tool for this?

/cc @roidrage @rkh