[medium] Add SECURITY.md with a private vulnerability reporting policy

SECURITY.md

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+# Security Policy
+
+## Supported versions
+
+Security fixes go into the latest release line (currently `v2.1.0`) and the `main` branch. Older releases are not patched; please upgrade.
+
+## Reporting a vulnerability
+
+**Please do not open a public issue, discussion or pull request for a security problem.**
+
+Report it privately through GitHub: **Security → Advisories → "Report a vulnerability"** on this repository (<https://github.com/SigmaHQ/pySigma-backend-splunk/security/advisories/new>).
+If you cannot use GitHub, contact one of the maintainers listed in `pyproject.toml` privately.
+
+Please include:
+
+- the version or commit, and a minimal Sigma rule, pipeline or command that reproduces the problem, with the observed and expected output;
+- the impact you see (see the scope below) and, if you have one, a proposed fix.
+
+## What we treat as a vulnerability
+
+This backend turns Sigma rules, which are often third-party or community content, into Splunk SPL / SPL2 queries and related output formats that run in a detection platform. The following are in scope:
+
+- **Query or output injection:** rule content (values, field names, titles, descriptions, tags or other metadata) that can change the *structure* of a generated query or of any output format this backend produces, instead of being treated as data. For example, breaking out of a quoted value, adding commands, clauses or sub-searches, or altering the fields of a generated configuration or alert definition.
+- **Code execution or file access:** converting a rule or loading a pipeline shipped with this backend that leads to code execution, unsafe deserialisation, template evaluation of untrusted input, or file access outside the intended location.
+- **Denial of service:** a small crafted rule that makes conversion consume excessive CPU or memory.
+- **Release and supply chain:** weaknesses in this repository's CI or release workflows that could let an outsider publish or alter a released package.
+
+**Out of scope (report publicly as bugs):**
+
+- Conversion or mapping errors that make a rule match more or fewer events than intended, without attacker-controlled input changing the structure of the output. These are important, but they are handled as normal issues and pull requests so fixes reach users quickly.
+- A systemic and severe detection gap may be reported privately; the maintainers decide whether it becomes an advisory.
+
+Report problems whose root cause is in pySigma itself (rule parsing, modifiers, processing pipeline machinery, conversion base classes) to [SigmaHQ/pySigma](https://github.com/SigmaHQ/pySigma). Report problems specific to this repository here.
+
+## Our process
+
+| Step                                                                          | Target                                        |
+| ----------------------------------------------------------------------------- | --------------------------------------------- |
+| Acknowledge the report                                                        | within 5 working days                         |
+| Initial assessment and severity (CVSS 3.1)                                    | within 14 days                                |
+| Fix developed in the advisory's temporary private fork                        | as soon as practical, normally within 90 days |
+| Coordinated release, then GitHub Security Advisory published (CVE via GitHub) | at the fix release                            |
+
+- We credit reporters in the advisory unless they ask not to be credited.
+- When a fix affects other SigmaHQ projects, we may coordinate their releases.
+- We ask reporters to keep details private until the advisory is published or 90 days have passed, whichever comes first, unless agreed otherwise.