doc: fix release notes section headers

Signed-off-by: David Aguilar <davvid@gmail.com>
README.md: add full-stops to make the argparse section easier to read

Signed-off-by: David Aguilar <davvid@gmail.com>
git-cola v2.0.0

Signed-off-by: David Aguilar <davvid@gmail.com>