docs: document how to load files from another server (#3622)

Fixes #1486

docs/config/02-files.md

@@ -1,28 +1,14 @@
-**The `files` array determines which files are included in the browser and which files are watched and served by Karma.**
+The `files` array determines which files are included in the browser, watched, and served by Karma.
 
-
-## Pattern matching and `basePath`
-- All of the relative patterns will get resolved using the `basePath` first.
-- If the `basePath` is a relative path, it gets resolved to the
-  directory where the configuration file is located.
-- Eventually, all the patterns will get resolved into files using
-  [glob], so you can use [minimatch] expressions like `test/unit/**/*.spec.js`.
-
-
-## Ordering
-- The order of patterns determines the order in which files are included in the browser.
-- Multiple files matching a single pattern are sorted alphabetically.
-- Each file is included exactly once. If multiple patterns match the
-  same file, it's included as if it only matched the first pattern.
-
-
-## Included, served, watched
-Each pattern is either a simple string or an object with the following properties:
+## `files`
+**Type.** Array
+**No Default.** This property is mandatory.
+**Description.** Each item is either a string (equivalent to `{ pattern: "<string>" }`) or an object with the following properties:
 
 ### `pattern`
 * **Type.** String
 * **No Default.** This property is mandatory.
-* **Description.** The pattern to use for matching.
+* **Description.** The pattern to use for matching. See below for details on how patterns are resolved.
 
 ### `type`
 * **Type.** String
@@ -68,6 +54,18 @@ Each pattern is either a simple string or an object with the following propertie
 * **Default.** `false`
 * **Description.** Should the files be served from disk on each request by Karma's webserver?
 
+## Pattern matching and `basePath`
+- All of the relative patterns will get resolved using the `basePath` first.
+- If the `basePath` is a relative path, it gets resolved to the
+  directory where the configuration file is located.
+- Eventually, all the patterns will get resolved into files using
+  [glob], so you can use [minimatch] expressions like `test/unit/**/*.spec.js`.
+
+## Ordering
+- The order of patterns determines the order in which files are included in the browser.
+- Multiple files matching a single pattern are sorted alphabetically.
+- Each file is included exactly once. If multiple patterns match the
+  same file, it's included as if it only matched the first pattern.
 
 ## Preprocessor transformations
 Depending on preprocessor configuration, be aware that files loaded may be transformed and no longer available in
@@ -100,6 +98,30 @@ files: [
 ],
 ```
 
+## Loading files from another server
+
+Pattern can also be an absolute URL. This allows including files which are not served by Karma.
+
+Example:
+
+```javascript
+config.set({
+  files: [
+    'https://example.com/my-lib.js',
+    { pattern: 'https://example.com/my-lib', type: 'js' }
+  ]
+})
+```
+
+Absolute URLs have some special rules comparing to the regular file paths:
+
+- Globing is not support, so each URL must be specified as a separate pattern.
+- Most of the regular options are not supported:
+    - `watched` is always `false`
+    - `included` is always `true`
+    - `served` is always `false`
+    - `nocache` is always `false`
+
 ## Loading Assets
 By default all assets are served at `http://localhost:[PORT]/base/`