Add reference to ignore options for addWatchTarget

A change proposed to solve 11ty#464
See also 11ty/eleventy#893

docs/config.md

@@ -523,6 +523,8 @@ module.exports = function(eleventyConfig) {
 };
 ```
 
+Eleventy will not add a watch for files or folders that are in `.gitignore`, unless `setUseGitIgnore` is turned off. See the chapter on [ignore files](/docs/ignores/#opt-out-of-using-.gitignore).
+
 ### Override Browsersync Server Options {% addedin "0.7.0" %}
 
 Useful if you want to change or override the default Browsersync configuration. Find the Eleventy defaults in [`EleventyServe.js`](https://github.com/11ty/eleventy/blob/master/src/EleventyServe.js). Take special note that Eleventy does not use Browsersync’s watch options and trigger reloads manually after our own internal watch methods are complete. See full options list on the [Browsersync documentation](https://browsersync.io/docs/options).