Add new asset process to README

[trmartin4]

📔 Objective

Updating the process for managing email assets now that we support using git for uploading assets.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Checkmarx One – Scan Summary & Details – 86f36d91-ec13-4c5e-a1ea-2910811d6806

Great job! No new security vulnerabilities introduced in this pull request

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.46%. Comparing base (b03f8f8) to head (25b00c3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7012   +/-   ##
=======================================
  Coverage   56.46%   56.46%           
=======================================
  Files        1996     1996           
  Lines       88039    88039           
  Branches     7845     7845           
=======================================
  Hits        49715    49715           
  Misses      36487    36487           
  Partials     1837     1837           

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