Updated Exports Files (markdown)

Exports-Files.md

@@ -60,6 +60,20 @@ The `generate-exports --typedef` command generate typedef's for the object liter
 
 Externs define types for options objects created by library's users, while typedef's define types created internally by the library.
 
+## The oli.js externs file
+
+The `externs/oli.js` file defines externs for functions that may be defined in application code and called by the library. For the library to be able to call user-defined functions, these functions should not be renamed within the library.
+
+For example, `oli.js` includes the following definition:
+
+    /**
+     * @param {ol.Map} map Map.
+     * @return {undefined} Undefined.
+     */
+    oli.control.Control.prototype.setMap = function(map) {};
+
+This will prevent the Compiler from renaming `setMap` internally, allowing the creation of custom controls defining their own `setMap` functions. (The `ol.control.Control.prototype.setMap` function is also exported, making it possible to call it from child implementations.)
+
 ## Under the hood
 
 `build/src/internal/src/requireall.js` is used in the `build.py build-all` target to ensure that all source files are passed to the compiler.