message: [Danger] Added rules to dangerfile
diff: /* @flow */
  -import { danger, fail } from 'danger'
  +import { danger, fail, warn } from 'danger'
  +import { includes } from 'lodash'
  // CHANGELOG check
  const modifiedAppFiles = danger.git.modified_files.filter(path => path.includes('lib/'))
  @@ -10,6 +11,24 @@ if (modifiedAppFiles.length > 0 && !changelogChanges) {
  fail('No CHANGELOG added.')
  }
  +if (!danger.github.pr.body.length) {
  + fail('Please add a description to your PR.')
  +}
  +
  +// Warns if there are changes to package.json without changes to yarn.lock.
  +const packageChanged = includes(danger.git.modified_files, 'package.json')
  +const lockfileChanged = includes(danger.git.modified_files, 'yarn.lock')
  +if (packageChanged && !lockfileChanged) {
  + const message = 'Changes were made to package.json, but not to yarn.lock'
  + const idea = 'Perhaps you need to run `yarn install`?'
  + warn(`${message} - <i>${idea}</i>`)
  +}
  +
  +const someoneAssigned = danger.github.pr.assignee
  +if (someoneAssigned === null) {
  + fail('Please assign someone to merge this PR, and optionally include people who should review.');
  +}
  +
  // Danger JS doesn't support warn yet.
  // const testFiles = _.filter(git.modified_files, function(path) {
  