Add advisory for another comrak XSS (#902)

Thanks to Sam Sanoop (snoopysecurity) for reporting.

Signed-off-by: Asherah Connor <ashe@kivikakk.ee>

crates/comrak/RUSTSEC-0000-0000.md

@@ -0,0 +1,26 @@
+```toml
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "comrak"
+date = "2021-05-04"
+url = "https://github.com/kivikakk/comrak/releases/tag/0.10.1"
+categories = ["format-injection"]
+keywords = ["xss"]
+
+[versions]
+patched = [">= 0.10.1"]
+```
+
+# XSS in `comrak`
+
+[comrak](https://github.com/kivikakk/comrak) operates by default in a "safe"
+mode of operation where unsafe content, such as arbitrary raw HTML or URLs with
+non-standard schemes, are not permitted in the output.  This is per the
+reference GFM implementation, [cmark-gfm](https://github.com/github/cmark).
+
+Ampersands were not being correctly escaped in link targets, making it possible
+to fashion unsafe URLs using schemes like `data:` or `javascript:` by entering
+them as HTML entities, e.g. `&#x64&#x61&#x74&#x61&#x3a`.  The intended
+behaviour, demonstrated upstream, is that these should be escaped and therefore
+harmless, but this behaviour was broken in comrak.
+