Add copy-pastable build instructions

[matklad]

The main value here is to put CMAKE_GENERATOR=Ninja front-and-center, so that users realize that extra steps are needed for a parallel build.

It is documented quite clearly already, but, from my very personal experience:

• people don't read the docs and spend hours hammering one poor core
• even if people do read the docs, when the come back to the repo the second time around, they forget, and hammer that single poor core again :-)

[daurnimator]

This makes it look like ninja is a requirement rather than an option.

people don't read the docs and spend hours hammering one poor core

Perhaps we could put CMAKE_BUILD_PARALLEL_LEVEL=$(nproc) instead?

[matklad]

I think ninja is a better default (simpler, faster, consistent across platforms), so, given that we already need CMake, nudging towards ninja seems like the right call.

[LinuxUserGD]

Maybe also add Windows Build Instructions to TL;DR?