Add SECURITY.md

Author: HadrienG2

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+This project aims for strict [SemVer](https://semver.org/) compliance. This
+means that upgrading to a new minor or patch version should be trivial, but
+upgrading to a new major version may incur some significant costs on your side.
+
+With that in mind, if we denote major.minor.patch the project's version
+number, the policy for security updates support is that...
+
+- The latest major.minor.patch version published on crates.io is supported
+  (obviously).
+- Each previously published major release remains supported, for users of the
+  latest minor.patch version, during 6 months after the release of the next
+  major release.
+
+
+## Reporting a Vulnerability
+
+### Basics
+
+Please do not report security vulnerabilities (any bug that has security
+implications if e.g. a setuid binary happens to use `triple-buffer`) using
+public channels such as...
+
+- A project's issue tracker
+- Social media
+- A [CVE
+  Report](https://en.wikipedia.org/wiki/Common_Vulnerabilities_and_Exposures)
+
+Doing so is the computer security equivalent of reporting a fire safety risk by
+starting a fire: you are informing attackers that software has a problem before
+a fix is available, and they will be able to exploit this information for the
+entire duration it will take us to come up with a fix, which is [Very
+Bad](https://en.wikipedia.org/wiki/Zero-day_(computing)).
+
+Instead, you should privately report the vulnerability to the project, give it
+some time to come up with a fix, and only report the vulnerability publicly once
+a patch release is available for all supported major versions (except in
+worst-case scenarios discussed below).
+
+### Reporting guidelines
+
+Please privately submit us a security advisory using [this dedicated
+form](https://github.com/HadrienG2/triple-buffer/security/advisories/new). A
+good security advisory should...
+
+- Describe the vulnerability
+- Assess its impact on downstream users of the library
+- Provides instructions on how to replicate the issue locally
+- Additionally provide a patch that addresses the issue, if you have one
+  available (otherwise the project will take care of writing it)
+
+### Worst-case scenarios
+
+While private reporting and embargo until a bugfix is available is the preferred
+course of action on our side, we are well aware that if we spend too much time
+without fixing the vulnerability, or if you become aware that another attacker
+has found out about the vulnerability and started exploiting it, you may want to
+publicly report the vulnerability anyway.
+
+Here's what you can expect from us in terms of response time :
+
+- At the time of writing, the project only has one permanent maintainer,
+  [**@HadrienG2**](https://github.com/HadrienG2). I will aim for fast replies
+  (less than a week, ideally next-day), but I may be momentarily unavailable
+  for various reasons: holidays, sickness... So please ping me a few times
+  (e.g. after a week and a month) before concluding that I am unresponsive and
+  must have abandoned the project.
+- Unless the vulnerability is extremely complex and requires a major library
+  rewrite to fix, 90 days should be more than enough time for me to fix it.