message: contributing: replace gitter with slack
  It looks like slack is now used, so gitter shouldn't be pointed to.
diff: @@ -3,7 +3,7 @@ maximize the chances of your PR being merged.
  # Communication
  -* Before starting work on a major feature, please reach out to us via GitHub, Gitter,
  +* Before starting work on a major feature, please reach out to us via GitHub, Slack,
  email, etc. We will make sure no one else is already working on it and ask you to open a
  GitHub issue.
  * A "major feature" is defined as any change that is > 100 LOC altered (not including tests), or
  @@ -98,7 +98,7 @@ maximize the chances of your PR being merged.
  organization different from the PR author.* E.g., if Lyft authors a PR, at least one approver
  should be from an organization other than Lyft. This helps us make sure that we aren't putting
  organization specific shortcuts into the code.
  -* If there is a question on who should review a PR please discuss in Gitter.
  +* If there is a question on who should review a PR please discuss in Slack.
  * Anyone is welcome to review any PR that they want, whether they are a committer or not.
  * Please **clean up the commit message** before merging. By default, GitHub fills the squash merge
  commit message with every individual commit from the PR. Generally, we want a commit message
  