Add GHC-67120 main not defined

Author: noughtmare

message-index/messages/GHC-67120/index.md

@@ -0,0 +1,16 @@
+---
+title: ‘main’ not defined
+summary: The IO action ‘main’ is not defined in module ‘Main’.
+introduced: 9.8.1
+severity: error
+---
+
+GHC expects the `Main` module to define a function called `main` which it can use as the entry point of your program.
+
+This error can also occur in unnamed modules, because GHC will default to the module name `Main` for such modules.
+
+If you just want GHC to produce an object file without an entry point, then you can give your module a name other than `Main` by putting a module header at the top of your file (below language pragmas and compiler options), for example as follows:
+
+```
+module Foo where
+```