Update readme instructions

[markf3lton]

Clarifications for installing phpunit and for running a single test from core/profiles/demo_umami

[markf3lton]

After chatting with @catch56, I confirmed there's a way to execute the tests without first ssh'ing into the container:

ddev exec phpunit -c web/core/ web/core/profiles/demo_umami/tests/src/FunctionalJavascript/OpenTelemetryNodePagePerformanceTest.php

And Claude recommends using DDEV custom commands by creating .ddev/commands/web/phpunit with

bash   #!/bin/bash
   cd /var/www/html/web && phpunit -c core/ "$@"

But, I like the instructions as-is.