This adds a linter to catch common markdown formatting errors.
The linter used is markdownlint-cli2 which is available on all platforms
and has an associated Github Action to automate these checks in CI.
Configuration is checked in at .markdownlint-cli2.yaml.
You may run the check locally using `markdownlint-cli2`.
Or you may install the extension for VSCode:
DavidAnson.vscode-markdownlint
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
