Adding instruction to service admin

[gillettmoj]

Purpose

Adding instruction to service admin readme, for how to add a new user to the admin portal

Fixes UML-3772

Approach

Explain how your code addresses the purpose of the change

Learning

Any tips and tricks, blog posts or tools which helped you. Plus anything notable you've discovered about the Use a Lasting Power of Attorney service

Checklist (tick/delete or strikethrough as appropriate)

• I have performed a self-review of my own code
• I have added tests to prove my work
• I have added relevant and appropriately leveled logging, without PII, to my code
• New event_codes have been documented on the wiki page
• I have updated documentation (Confluence/GitHub wiki/tech debt doc)
• I have added welsh translation tags and updated translation files
• I have run an accessibility tool on any pages I have made changes to and fixed any issues found
• I have notified the Interaction Designer of any content changes so that appropriate screenshots/flow diagram changes can be made
• The product team have tested these changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.85%. Comparing base (f00760a) to head (1c4d637).
Report is 205 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #3020       +/-   ##
=============================================
- Coverage     91.25%   78.85%   -12.40%     
=============================================
  Files           309       21      -288     
  Lines          6393      998     -5395     
=============================================
- Hits           5834      787     -5047     
+ Misses          542      194      -348     
  Partials         17       17               

Flag	Coverage Δ
use-an-lpa-admin	78.85% <ø> (-0.41%)	⬇️
use-an-lpa-api	?
use-an-lpa-front	?

see 305 files with indirect coverage changes