Added Markdownlint configuration

[jonasbn]

Hello @DanielKeep and others.

I was looking over the repository and my editor complained about the Markdown. I am using Markdownlint to check my Markdown as I write it. To avoid issues, I added a Markdownlint configuration and decided to share it with you.

If you have questions to the configuration or Markdownlint in generation I will gladly answer these. If you would like to get Markdownlint integrated with CI using Travis or similar please let me know.

The initial revision disables or configures Markdownlint rules to make the current source pass the Markdownlint check.

The following rules, should be addressed:

• no-trailing-spaces, should be easy, have a look at EditorConfig or similar

The following rules can just be kept in the specified configuration.

• first-line-heading, seems to conflict with the implementation, which uses % for indicating headings. (This is not-markdown) and is expected to be something local

• ul-indent, has been configured from the default of 2 to 4. This is primarily addressing issues in Summary.md

• line-length rule, has been configured to falseindicating disabled, since length of lines is a style issue. Could be addressed with EditorConfig or similar

Ref: https://github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md