Add note on how to test an extension directly from a PR

[derekbekoe]

e.g.

docker run -it microsoft/azure-cli:latest
pip install --upgrade --target ~/.azure/cliextensions/servicebus "git+https://github.com/v-Ajnava/azure-cli-extensions@SBmasterExt#subdirectory=src/servicebus&egg=servicebus"

Add this to the PR template.

[derekbekoe]

Created an Azure function bot that comments on PRs with this information automatically completed with the appropriate install URL.