List markdown as unmaintained (#1191)

* List `markdown` as unmaintained

The `markdown` crate is, naturally, the first one that comes up if you're searching crates.io for "markdown". Unfortunately, that particular crate has not received any updates since November of 2020 despite several known issues with open PRs. I opened johannhof/markdown.rs#48 to request an update on maintenance status nearing a month ago and have no heard anything back.

Author: dbanty

crates/markdown/RUSTSEC-0000-0000.md

@@ -0,0 +1,17 @@
+```toml
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "markdown"
+date = "2022-01-17"
+informational = "unmaintained"
+url = "https://github.com/johannhof/markdown.rs/issues/48"
+
+[versions]
+patched = []
+unaffected = []
+```
+
+# `markdown` is unmaintained
+
+The [`markdown`](https://crates.io/crates/markdown) crate is no longer actively maintained. For Markdown parsing, you can use the [pulldown-cmark](https://crates.io/crates/pulldown-cmark) crate.
+