Add a design doc template

[stephen-crawford]

Description

This change introduces a new markdown file for people looking to make major changes to the Security plugin.

It covers basic information about what a design doc should use and a tool for knowing what info is required.

Check List

• New functionality includes testing
• New functionality has been documented
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

Merging #3556 (dccc810) into main (ccc3e34) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #3556      +/-   ##
============================================
- Coverage     65.04%   65.00%   -0.04%     
+ Complexity     3650     3646       -4     
============================================
  Files           283      283              
  Lines         20619    20619              
  Branches       3392     3392              
============================================
- Hits          13411    13403       -8     
- Misses         5528     5536       +8     
  Partials       1680     1680              

see 3 files with indirect coverage changes

[peternied]

Thanks for spending the time to write this up, what do you think about quickly translating an existing feature into this template so we can see how it looks in practice?

[DarshitChanpura]

Thank you @scrawfor99 for adding this. One suggestion I'd make is to have a check-list of items necessary for documenting design. Maybe we can update the pull-request template to have one more section for design documentation? Thoughts?