Add a security policy. (#1193)

Motivation:

Security is important! We should let users know what to do if they find
or suspect they have found a vulnerability.

Modifications:

- Add SECURITY.md

Result:

Resolves #1114

README.md

@@ -146,6 +146,10 @@ The `docs` directory contains documentation, including:
 - Support for Apple Platforms and NIO Transport Services in
   [`docs/apple-platforms.md`][docs-apple]
 
+## Security
+
+Please see [SECURITY.md](SECURITY.md).
+
 ## License
 
 gRPC Swift is released under the same license as [gRPC][grpc], repeated in

SECURITY.md

@@ -0,0 +1,46 @@
+# Security
+
+This document specifies the security process for gRPC Swift.
+
+## Disclosures
+
+### Private Disclosure Process
+
+The gRPC Swift team asks that known and suspected vulnerabilities be
+privately and responsibly disclosed by emailing
+[grpc-swift-security@group.apple.com](mailto:grpc-swift-security@group.apple.com)
+with the [details usually included with bug reports][issue-template].
+**Do not file a public issue.**
+
+#### When to report a vulnerability
+
+* You think you have discovered a potential security vulnerability in gRPC
+  Swift.
+* You are unsure how a vulnerability affects gRPC Swift.
+
+#### What happens next?
+
+* A member of the team will acknowledge receipt of the report within 3
+  working days (United Kingdom). This may include a request for additional
+  information about reproducing the vulnerability.
+* We will privately inform the Swift Server Work Group ([SSWG][sswg]) of the
+  vulnerability within 10 days of the report as per their [security
+  guidelines][sswg-security].
+* Once we have identified a fix we may ask you to validate it. We aim to do this
+  within 30 days. In some cases this may not be possible, for example when the
+  vulnerability exists at the protocol level and the industry must coordinate on
+  the disclosure process.
+* If a CVE number is required, one will be requested from [MITRE][mitre]
+  providing you with full credit for the discovery.
+* We will decide on a planned release date and let you know when it is.
+* Prior to release, we will inform major dependents that a security-related
+  patch is impending.
+* Once the fix has been released we will publish a security advisory on GitHub
+  and the [SSWG][sswg] will announce the vulnerability on the [Swift
+  forums][swift-forums-sec].
+
+[issue-template]: https://github.com/grpc/grpc-swift/blob/main/.github/ISSUE_TEMPLATE/BUG_REPORT.md
+[sswg]: https://github.com/swift-server/sswg
+[sswg-security]: https://github.com/swift-server/sswg/blob/main/process/incubation.md#security-best-practices
+[swift-forums-sec]: https://forums.swift.org/c/server/security-updates/
+[mitre]: https://cveform.mitre.org/