docs: add a security policy

SECURITY.md

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+# Security policy
+
+## Reporting a vulnerability
+
+Report a vulnerability privately, through GitHub's
+[private vulnerability reporting](https://github.com/dexpace/morphic/security/advisories/new): on
+the repository's **Security** tab, choose **Report a vulnerability**. Do not open a public issue or
+pull request about it. The report opens a draft advisory that only you and the maintainers can
+read, and it is followed up with you there.
+
+A report is quickest to act on when it gives:
+
+- the Morphic commit you ran;
+- the command or library call, with the options it set;
+- the smallest spec that reproduces the problem, with every document the spec references;
+- what happened, and what you expected instead.
+
+If you are unsure whether something is a vulnerability, report it privately anyway.
+
+## Supported versions
+
+Morphic has no released version yet. Fixes land on `main`, so only its latest commit is supported:
+check that a report reproduces there.
+
+## Scope
+
+Morphic treats the spec it compiles, and every document the spec references, as untrusted. It
+trusts what its caller supplies: the options, a file an option names (an overlay), and the code a
+[library](README.md#library) caller hands it, such as a compiler, a pass or a cache.
+
+The [README](README.md#design-principles) states the bounds Morphic holds untrusted input to:
+
+- Unless `allow-external-refs` is set, a `$ref` reads only regular files inside the spec's
+  directory, through relative references. An absolute path, a `file:` URI, a URL and a symbolic
+  link out of the directory are each refused.
+- [Budgets](README.md#compiler-options) on what a document or a compile may cost turn a
+  pathological document into a diagnostic instead of an exhausted host.
+
+A vulnerability is untrusted input that gets past one of these bounds: a spec that makes Morphic
+read a file or reach a URL it should have refused, or a document that crashes the process, runs
+without end or exhausts its memory instead of ending in a diagnostic.
+
+Setting `allow-external-refs` says the spec is trusted to name the files and URLs it reads, so set
+it only for a spec you trust. A document fetched over the network stays untrusted: one that makes
+Morphic read a local file is a vulnerability.
+
+Two things are not vulnerabilities. What a document costs under a budget the caller raised or
+turned off is the caller's choice. Wrong or lossy IR for a valid spec is a bug: open a
+[public issue](https://github.com/dexpace/morphic/issues) for it.
+
+A weakness already filed under the
+[`security` label](https://github.com/dexpace/morphic/issues?q=label%3Asecurity) is known and
+public, so add to that issue rather than reporting it again.