NO-JIRA add README.txt file to explain the need of an empty PULL_REQUEST_TEMPLATE.md

.github/README.txt

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+The empty PULL_REQUEST_TEMPLATE.md file is here to override the one defined in /docs/PULL_REQUEST_TEMPLATE.md.
+
+This allows us to not show a template for PRs on the sonarsource/sonar-enterprise repository, while leaving a template for the public sonarsource/sonarqube repository.