fix the following markdownlint errors (and some more):
[..]mixins/node-exporter/README.md:13: MD031 Fenced code blocks should be surrounded by blank lines
[..]mixins/node-exporter/README.md:21: MD031 Fenced code blocks should be surrounded by blank lines
[..]mixins/node-exporter/README.md:27: MD031 Fenced code blocks should be surrounded by blank lines
[..]mixins/node-exporter/README.md:33: MD031 Fenced code blocks should be surrounded by blank lines
[..]mixins/node-exporter/README.md:41: MD034 Bare URL used
A detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md
Signed-off-by: Sven Kieske <s.kieske@mittwald.de>
