Add build instructions

[mwinters0]

CONTRIBUTING.md still references build instructions in README.md but they are no longer there.

Personally, I look for build info in the repo itself as it generally tends to stay more accurate. (Code review can ensure that any commit which changes build tooling also includes doc updates.) That said, if this PR is accepted I'll make a similar PR for the website repo.

[softwarefactory-project-zuul[bot]]

Build succeeded. https://softwarefactory-project.io/zuul/t/local/buildset/11a0e5c2aa13403292432a2767fd6dbb

:heavy_check_mark: unit-test SUCCESS in 5m 40s :heavy_check_mark: unit-test-migration-path-for-coreos-toolbox SUCCESS in 3m 27s :heavy_check_mark: unit-test-restricted SUCCESS in 5m 49s :heavy_check_mark: system-test-fedora-rawhide-commands-options SUCCESS in 1h 06m 29s :heavy_check_mark: system-test-fedora-rawhide-runtime-environment-arch-fedora SUCCESS in 52m 19s :heavy_check_mark: system-test-fedora-rawhide-runtime-environment-ubuntu SUCCESS in 14m 54s :heavy_check_mark: system-test-fedora-42-commands-options SUCCESS in 1h 04m 12s :heavy_check_mark: system-test-fedora-42-runtime-environment-arch-fedora SUCCESS in 55m 36s :heavy_check_mark: system-test-fedora-42-runtime-environment-ubuntu SUCCESS in 15m 13s :heavy_check_mark: system-test-fedora-41-commands-options SUCCESS in 1h 05m 49s :heavy_check_mark: system-test-fedora-41-runtime-environment-arch-fedora SUCCESS in 55m 57s :heavy_check_mark: system-test-fedora-41-runtime-environment-ubuntu SUCCESS in 15m 09s :heavy_check_mark: system-test-fedora-40-commands-options SUCCESS in 1h 06m 30s :heavy_check_mark: system-test-fedora-40-runtime-environment-arch-fedora SUCCESS in 55m 57s :heavy_check_mark: system-test-fedora-40-runtime-environment-ubuntu SUCCESS in 16m 21s