readme-plugin: Add motivation && installationSteps parsers

[dimitrisnl]

Change-type: minor

[dimitrisnl]

Test runs for Landr:

{
  installationSteps: {
    headers: [],
    steps: [ [Array], [Array], [Array] ],
    footer: [ [Array] ]
  },
  motivation: [
    'para',
    'Creating a website for a project is hard and time-consuming. ' +
      'Yet, a great\nwebsite is one of the best ways to promote a ' +
      'project. Landr aims to capture our\nexperience presenting a wide ' +
      'variety of open source projects on the web, so\nthat having a ' +
      'website that can compete with your favourite massively popular\n' +
      'open source project is not only possible, but comes with zero ' +
      'overhead.'
  ]
}