Add example of unknown module error (documentationjs#83)

Describe possible fix that you need to install a dependency with
`bower`.

Author: tchajed

errors/UnknownName.md

@@ -13,14 +13,24 @@ at line 1, column 13 - line 1, column 14
   Unknown value y
 ```
 
+```
+> import Math
+
+Error found:
+in module $PSCI
+
+  Unknown module Math
+```
+
 ## Cause
 
-This error if a name is used but that name is not in scope. Names can refer to various things - values, types, operators, type classes, etc.
+This error if a name is used but that name is not in scope. Names can refer to various things - values, types, modules, operators, type classes, etc.
 
 ## Fix
 
 - Verify the spelling of any names used.
 - Verify that the name has been imported.
 - If you are using qualified imports, make sure the module name is correct.
+- If an import results in an "Unknown module" error, make sure that any required dependencies have been installed.
 
 ## Notes