Fix Readme to account for doc reorganization
Fix Makefile options section
Fixes as per Mark's suggestions
Improve wording of top-level Makefile description
Additional fixes
build: Ignore _build directories
Merge pull request #332 from rleigh-dundee/updateignore-dev44

build: Ignore _build directories (dev_4_4)
Merge pull request #327 from sbesson/10687_README

Fix readme (see #10687)