Add subject line best practices to emails/nurture handbook

Why is this change being made?

To add overall recommendations for subject lines specifically. @nbsmith in looking at the results of the Commit emails, there were outstanding open rates. How can teams produce similar results? What are some of your recommendations that we can point people to?

Please feel free to link to other resources - the Corp team was hoping to learn from you and pointing them to the handbook may be a good setup (and we can point other email copy writing team members there as well!)

I started to write some things in then thought, I should probably leave this to the expert 😅

cc @Lconway @Emily @tshurtleff

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)