README.OE-Core.md: fix markdown style issues

README.OE-Core.md: fix markdown style issues

Fix markdown style issues using markdownlint tool [1]:

$ cat README.OE-Core.md | mdl
(stdin):15: MD012 Multiple consecutive blank lines
(stdin):9: MD034 Bare URL used
(stdin):13: MD034 Bare URL used
(stdin):19: MD034 Bare URL used
(stdin):25: MD046 Code block style
(stdin):29: MD046 Code block style
(stdin):33: MD046 Code block style

[1] https://github.com/markdownlint/markdownlint
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>