Replace outdated instructions in README

[n0toose]

The commit 8ceeb6c introduced some documentation 5 years ago and has been out of date for the past year because of a refactor that e.g. removed the file ./haikuporter/buildmaster/bin/bootstrap_buildmaster.sh. The commit in question is 607b43b.

How things should be done nowadays is not immediately clear -- that prevents porters from e.g. creating new commits on their preferred development environment.

[n0toose]

I would've gladly made a PR to fix this myself, as I believe that the inability to get a portable environment that allows me to get back into contributing, fast, when I treat my Haiku installations as disposable (e.g. distro hopping, changing devices, etc.) has affected my personal output -- and this has probably affected other (prospective) contributors as well.