HOWTO_UNMAINTAINED.md: guide for unmaintained crate advisories (#1192)

Initial guide on policy around what RustSec considers to be an
unmaintained crate as well as the policy for filing an advisory

HOWTO_UNMAINTAINED.md

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+# HOWTO Guide: Unmaintained Crate Advisories
+
+This document describes the policy for adding advisories for unmaintained
+crates to the [RustSec Advisory Database].
+
+These advisories serve to inform the Rust community about both the existence
+of unmaintained crates within a particular project, and also serve to guide
+switching to maintained alternatives.
+
+## Definition of an "unmaintained" crate
+
+We consider crates unmaintained when they fall into either of the following
+categories:
+
+- Explicitly unmaintained: a crate's author has declared that they are no
+  longer maintaining a particular crate.
+- Implicitly unmaintained: the author is incommunicado for a prolonged period
+  of time and cannot advise as to a crate's status.
+
+## Creating an unmaintained crate advisory
+
+### Policy
+
+When in doubt, we always defer to the author of a crate's discretion as to
+whether they would prefer an unmaintained crate advisory be filed, provided
+we are able to make contact.
+
+First and foremost: *ASK THE AUTHOR(S)*, preferably in a public issue on the
+project's source code repository. If an author/maintainer of a particular crate
+thinks filing an unmaintained crate advisory is a good idea, then great! Go ahead.
+
+If the author is responsive and declares that the project *is* maintained, then
+the RustSec organization considers it maintained and won't accept advisories about its maintenance status. Again, when in doubt, defer
+to the author's discretion. So long as the author is responsive and avows that
+a crate is maintained, we take them at their word. Repository metrics like
+recent commits, open issues, latest release, etc are not reasons to go against
+the direct word of a crate author.
+
+However, if attempts have been made to contact a crate author have failed,
+metrics like recent commits, open issues, time since last crate release etc
+are important evidence to justify that a crate is unmaintained. An
+incommunicado crate author is irrelevant if there is evidence that work is
+continuing to happen on a crate.
+
+To justify the "implicitly unmaintained" status, where a crate author is
+unreachable, the following criteria must be met:
+
+- Stale repository: no recent maintenance activity, including any of the
+  following: recent commits, responses from the author on open issues,
+  crate releases, or other publically visible activity by the author.
+  Inactivity over a period of 1 year or more is the preferred threshold.
+- Contact attempts with the author made with no response. Ideally these
+  attempts are made via a public GitHub issue, so that issue can be
+  cited in an unmaintained crate advisory if need be. Unresponsiveness
+  by the author over a period of 90 days is suggested before filing an
+  advisory.
+
+### Process
+
+Unmaintained crate advisories use the same structure as RustSec security
+advisories, but include an `informational = "unmaintained"` attribute in
+the TOML advisory.
+
+When creating the advisory, please include a link to an open issue
+on the upstream project repository where the maintenance status has been
+discussed in the `url = "..."` field of the advisory.
+
+For more information on adding an advisory to the RustSec DB, see:
+
+<https://github.com/RustSec/advisory-db/blob/main/CONTRIBUTING.md>
+
+### Questions
+
+Please open a GitHub issue:
+
+<https://github.com/rustsec/advisory-db/issues>
+
+[//]: # (links)
+
+[RustSec Advisory Database]: https://rustsec.org