docs: add security policy

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+If you believe you've found a security vulnerability in Selenium Boot, please report it
+privately — **do not open a public GitHub issue.**
+
+Email **security@seleniumboot.com** with:
+
+- a description of the issue and its impact,
+- the version affected (`mvn dependency:tree` or your `pom.xml`),
+- steps to reproduce, and a proof of concept if you have one.
+
+You can expect an initial acknowledgement within **72 hours**. We'll keep you updated as
+we investigate, and we'll credit you in the release notes once a fix ships (unless you'd
+prefer to remain anonymous).
+
+Please give us a reasonable window to release a fix before any public disclosure.
+
+## Supported versions
+
+Selenium Boot is pre-1.0 and moving quickly. Security fixes are applied to the **latest
+released version** on [Maven Central](https://central.sonatype.com/artifact/io.github.seleniumboot/selenium-boot).
+Please make sure you're on the latest release before reporting.
+
+## Scope
+
+Selenium Boot is a test-automation framework — it runs as part of your build, driving
+browsers you control. The most relevant classes of issue are, for example: unsafe
+handling of configuration or test data, dependency vulnerabilities, or anything that
+could let untrusted input escalate during a test run. Reports about the framework's own
+code and its published artifacts are in scope; issues in Selenium, WebDriver, or browser
+binaries themselves should go to their respective projects.