Fix #616 document hook_civicrm_getAssetUrl

docs/hooks/hook_civicrm_getAssetUrl.md

+# hook_civicrm_getAssetUrl
+
+## Summary
+
+This hook is called when building a link to a semi-static asset, allowing you to modify the params the asset will be built with.
+
+## Notes
+
+For more discussion, see [AssetBuilder](/framework/asset-builder.md).
+
+## Definition
+
+    hook_civicrm_getAssetUrl(&$asset, &$params)
+
+## Parameters
+
+ * `$asset` (string): the logical file name of an asset (ex: `hello-world.json`)
+ * `$params` (array): an optional set of parameters describing how to build the asset
+
+## Returns
+
+ * null
+
+## Example
+
+```php
+function mymodule_civicrm_getAssetUrl(&$asset, &$params) {
+  if ($asset === 'hello-world.json') {
+    $params['planet'] = 'Earth';
+  }
+}
+```