doc: Improve cmake instructions in README

[fjahr]

Minor improvement suggestion for the readme. I find this alternative way of using cmake a bit more comfortable because I don't like to change the directory.

It's just a suggestion based on personal preference, if this is too minor of an improvement feel free to close.

[real-or-random]

I happen to find this a tiny bit more comfortable, too, but IIRC we had discussed this (I can't find the discussion) and @hebasto argued that the tutorial cds into the build directory. I also think that mkdir build && cd build is a bit more self-documentary to people not familiar with CMake because it makes clear that build is a directory and not some CMake command or something.

No strong opinion from my side, but I think there are good reasons for what we have currently.

[fjahr]

@real-or-random alright, thanks for the info, closing it for now

[hebasto]

FWIW, while working on the CMake staging branch for Bitcoin Core, the rough consensus has been reached not to suggest using cd in the build docs (see https://github.com/bitcoin/bitcoin/blob/master/doc/build-*.md).