Add instructions for the Closure externs generator

docs/closure_compilation.md

 # Closure Compilation
 
 ## What is type safety?
 
 [Strongly-typed languages](https://en.wikipedia.org/wiki/Strong_and_weak_typing)
 like C++ and Java have the notion of variable types.
 
 This is typically baked into how you declare variables:
 
 ```c++
@@ skipping 178 matching lines @@
 ++       '../my_project/compiled_resources2.gyp:*',
       ],
     }
   ]
 }
 ```
 
 This file is used by the
 [Closure compiler bot](https://build.chromium.org/p/chromium.fyi/builders/Closure%20Compilation%20Linux)
 to automatically compile your code on every commit.
+
+## Externs
+
+[Externs files](https://github.com/google/closure-compiler/wiki/FAQ#how-do-i-write-an-externs-file)
+define APIs external to your JavaScript. They provide the compiler with the type
+information needed to check usage of these APIs in your JavaScript, much like
+forward declarations do in C++.
+
+Third-party libraries like Polymer often provide externs. Chrome must also
+provide externs for its extension APIs. Whenever an extension API's `idl` or
+`json` schema is updated in Chrome, the corresponding externs file must be
+regenerated:
+
+```shell
+./tools/json_schema_compiler/compiler.py -g externs \
+  extensions/common/api/your_api_here.idl \
+  > third_party/closure_compiler/externs/your_api_here.js
+```