Update guidelines re: DOI

[walshbr]

Closes #1776. Needs translation into Fr, Es, Pt. It should be ready to be translated.

Checklist

• Assign yourself in the "Assignees" menu
• Assign at least one individual or team to "Reviewers"
  • if the text needs to be translated, assign the relevant language team(s) as "Reviewers" and tag both the team as well as the managing edtor in your PR. Please follow the translation request guidelines when writing your PR description
• Add the appropriate "Label"
• Ensure the Travis CI checks pass
• Check the live preview of your PR on Netlify
• If this PR closes an open issue, add the phrase Closes #ISSUENUMBER to the description above

If you are having difficulty fixing Travis errors, first consult https://github.com/programminghistorian/jekyll/wiki/Making-Technical-Contributions carefully, especially "Common Travis Errors". Then contact the technical team if you need further help.

[walshbr]

Ready for translation @programminghistorian/french-team @programminghistorian/spanish-team @programminghistorian/portuguese-team! Let me know if you have any questions.

[walshbr]

@rivaquiroga - were you done with your changes? If so, I think the @programminghistorian/french-team review is the only one outstanding.

[rivaquiroga]

@walshbr, yes, I'm done.
My approval went stale after Daniel's commits.