Markdownlint configuration

[jonasbn]

Hi @naokazuterada

I have added a basic Markdownlint configuration and adjusted a few Markdown issues pointed out by Markdownlint.

I am primarily using the plugin for VSCode, but there is both a Node and a Ruby implementation.

A marvelous tool to catch basic Markdown issues and helping in following best practices.

[codecov]

Codecov Report

Merging #133 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #133   +/-   ##
=======================================
  Coverage   90.44%   90.44%           
=======================================
  Files           8        8           
  Lines         408      408           
=======================================
  Hits          369      369           
  Misses         39       39

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[naokazuterada]

@jonasbn Thanks!!