Updates the README for the new location permissions.

[mvanbeusekom]

Updates the README with documentation on how to use the PERMISSION_LOCATION_WHENINUSE macro on iOS.

Pre-launch Checklist

• I made sure the project builds.
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is does not need version changes.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I rebased onto main.
• I added new tests to check the change I am making, or this PR does not need tests.
• I made sure all existing and new tests are passing.
• I ran dart format . and committed any changes.
• I ran flutter analyze and fixed any errors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.42%. Comparing base (412f25c) to head (76ebc8b).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1297   +/-   ##
=======================================
  Coverage   71.42%   71.42%           
=======================================
  Files           1        1           
  Lines          35       35           
=======================================
  Hits           25       25           
  Misses         10       10           

Flag	Coverage Δ
unittests	71.42% <ø> (ø)

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[TimHoogstrate]

LGTM