Please describe what prettier ignores by default (#12626)

Co-authored-by: Szymon Dzialowski <sdzialowski@godaddy.com>
Co-authored-by: Szymon Dzialowski <sdzialowski@gmail.com>
Co-authored-by: fisker Cheung <lionkay@gmail.com>

docs/ignore.md

@@ -24,6 +24,27 @@ coverage
 
 It’s recommended to have a `.prettierignore` in your project! This way you can run `prettier --write .` to make sure that everything is formatted (without mangling files you don’t want, or choking on generated files). And – your editor will know which files _not_ to format!
 
+By default prettier ignores files in version control systems directories (".git", ".svn" and ".hg") and `node_modules` (if [`--with-node-modules` CLI option](cli.md#--with-node-modules) not specified)
+
+So by default it will be
+
+```js
+**/.git
+**/.svn
+**/.hg
+**/node_modules
+```
+
+and
+
+```js
+**/.git
+**/.svn
+**/.hg
+```
+
+if [`--with-node-modules` CLI option](cli.md#--with-node-modules) provided
+
 (See also the [`--ignore-path` CLI option](cli.md#--ignore-path).)
 
 ## JavaScript