Enhance README with explanation concerning issue #167.

README.markdown

@@ -114,6 +114,9 @@ Here is an example:
 
 ```haskell
 module Foo where
+
+import Bar.Baz
+
 -- $setup
 -- >>> let x = 23 :: Int
 
@@ -124,6 +127,12 @@ foo :: Int
 foo = 42
 ```
 
+Note that you should not place setup code inbetween the module header (`module
+...  where`) and import declarations. GHC will not be able to parse it (issue
+ #167). It is best to place setup code right after import declarations, but due
+to its declarative nature you can place it anywhere inbetween top level
+declarations as well.
+
 
 ### Multi-line input
 GHCi supports commands which span multiple lines, and the same syntax works for doctest: