Mention that build-tools can refer to locally defined executables.

Signed-off-by: Edward Z. Yang <[email protected]>

Cabal/doc/developing-packages.markdown

@@ -1413,8 +1413,10 @@ for these fields.
 
 `build-tools:` _program list_
 :   A list of programs, possibly annotated with versions, needed to
-    build this package, e.g. `c2hs >= 0.15, cpphs`.If no version
+    build this package, e.g. `c2hs >= 0.15, cpphs`.  If no version
     constraint is specified, any version is assumed to be acceptable.
+    `build-tools` can refer to locally defined executables, in which
+    case Cabal will make sure that executable is built first.
 
 `buildable:` _boolean_ (default: `True`)
 :   Is the component buildable? Like some of the other fields below,