gnucash-docs maint: Update README for cmake build system

[Geert Janssens]

Updated	 via  https://github.com/Gnucash/gnucash-docs/commit/a001c20e (commit)
	from  https://github.com/Gnucash/gnucash-docs/commit/cad4c52f (commit)



commit a001c20eafbac6b3efc0af1c9a0f92c0c8cc39f0
Author: Geert Janssens <geert at kobaltwit.be>
Date:   Tue Feb 9 14:00:08 2021 +0100

    Update README for cmake build system



Summary of changes:
 README | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 54 insertions(+), 10 deletions(-)