docs: Add missing link to hang failure docs

Author: sdboyer

docs/failure-modes.md

@@ -55,7 +55,7 @@ The exact error text will vary depending on which of the operations is running,
 
 Almost any case where a dep command, run with `-v`, hangs for more than ten minutes will ultimately be a bug. However, the most common explanation for an apparent dep hangs is actually normal behavior: because dep's operation requires that it keep its own copies of upstream sources hidden away in the [local cache](glossary.md#local-cache), the first run of dep against a project, especially large projects, can take a long time while it populates the cache.
 
-The only known case where dep may hang indefinitely is if one of the underlying VCS binaries it calls is prompting for some kind of input. Typically this means credentials (though not always - make sure to accept remote hosts' SSH keys into your known hosts!), and dep's normal assumption is that necessary credentials have been provided via environmental mechanisms - [configuration files or daemons](FAQ.md#how-do-i-get-dep-to-authenticate-to-a-git-repo), SSH agents, etc. This assumption is necessary for dep's concurrent network activity to work. If your use case absolutely cannot support the use of any such environmental caching mechanism, [please weigh in on this issue]().
+The only known case where dep may hang indefinitely is if one of the underlying VCS binaries it calls is prompting for some kind of input. Typically this means credentials (though not always - make sure to accept remote hosts' SSH keys into your known hosts!), and dep's normal assumption is that necessary credentials have been provided via environmental mechanisms - [configuration files or daemons](FAQ.md#how-do-i-get-dep-to-authenticate-to-a-git-repo), SSH agents, etc. This assumption is necessary for dep's concurrent network activity to work. If your use case absolutely cannot support the use of any such environmental caching mechanism, [please weigh in on this issue](https://github.com/golang/dep/issues/1476).
 
 Unfortunately, until dep [improves the observability of its ongoing I/O operations](), it cannot accurately report to the user which operations are actually underway at any given moment. This can make it difficult to differentiate from other hangs - credentials prompts, long network timeouts induced by firewalls, sluggish TCP when faced with packet loss, etc.