Add instanced data topic

Author: syreal17

topics/README.md

@@ -9,3 +9,5 @@ challenges and provided short answers with examples of how to do certain things.
 - [Adding downloadable artifact](/topics/adding-artifact.md). Most challenges
   will require interacting with a file artifact. Here's how to present artifacts
   to players.
+- [Adding extra instanced data](/topics/instanced-data.md). Some challenges
+  require extra instance-specific data, like an SSH password.

topics/instanced-data.md

@@ -0,0 +1,8 @@
+# Adding extra instanced data
+
+Adding extra instanced data that can be presented in the problem description
+involves two steps:
+
+1. Adding the data to `/challenge/metadata.json`, see
+   [here](/example-problems/general-ssh/config-builder.py#L66)
+1. Linking to the data in the problem description, see [here](/example-problems/general-ssh/problem.md?plain=1#L18)