Document performance implications of gitignore option

Fixes #50

Author: sindresorhus

index.d.ts

@@ -42,6 +42,8 @@ export type Options = {
 	/**
 	Respect ignore patterns in `.gitignore` files that apply to the globbed files.
 
+	Performance note: This option searches for all `.gitignore` files in the entire directory tree before globbing, which can be slow. For better performance, use `ignoreFiles: '.gitignore'` to only respect the root `.gitignore` file.
+
 	@default false
 	*/
 	readonly gitignore?: boolean;
@@ -51,6 +53,8 @@ export type Options = {
 
 	This is a more generic form of the `gitignore` option, allowing you to find ignore files with a [compatible syntax](http://git-scm.com/docs/gitignore). For instance, this works with Babel's `.babelignore`, Prettier's `.prettierignore`, or ESLint's `.eslintignore` files.
 
+	Performance tip: Using a specific path like `'.gitignore'` is much faster than recursive patterns.
+
 	@default undefined
 	*/
 	readonly ignoreFiles?: string | readonly string[];

readme.md

@@ -86,6 +86,8 @@ Default: `false`
 
 Respect ignore patterns in `.gitignore` files that apply to the globbed files.
 
+**Performance note:** This option searches for *all* `.gitignore` files in the entire directory tree before globbing, which can be slow. For better performance, use `ignoreFiles: '.gitignore'` to only respect the root `.gitignore` file.
+
 ##### ignoreFiles
 
 Type: `string | string[]`\
@@ -95,6 +97,8 @@ Glob patterns to look for ignore files, which are then used to ignore globbed fi
 
 This is a more generic form of the `gitignore` option, allowing you to find ignore files with a [compatible syntax](http://git-scm.com/docs/gitignore). For instance, this works with Babel's `.babelignore`, Prettier's `.prettierignore`, or ESLint's `.eslintignore` files.
 
+**Performance tip:** Using a specific path like `'.gitignore'` is much faster than recursive patterns.
+
 ### globbySync(patterns, options?)
 
 Returns `string[]` of matching paths.