gnucash maint: Make ChangeLog.2020, generate new ChangeLog beginning 2021.

[John Ralls]

Updated	 via  https://github.com/Gnucash/gnucash/commit/16af91b8 (commit)
	from  https://github.com/Gnucash/gnucash/commit/2d456913 (commit)



commit 16af91b8049f30e2eb8acc56b771ef9875c6d099
Author: John Ralls <jralls at ceridwen.us>
Date:   Fri Mar 26 16:10:24 2021 -0700

    Make ChangeLog.2020, generate new ChangeLog beginning 2021.



Summary of changes:
 CMakeLists.txt |    2 +-
 ChangeLog.2020 | 6464 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 6465 insertions(+), 1 deletion(-)
 create mode 100644 ChangeLog.2020