show how to exclude individual files in the exclude example (#663)

* show how to exclude individual files in the exclude example

* include comments in the regex

README.md

@@ -572,24 +572,24 @@ line-length = 88
 py36 = true
 include = '\.pyi?$'
 exclude = '''
-/(
-    \.eggs
-  | \.git
-  | \.hg
-  | \.mypy_cache
-  | \.tox
-  | \.venv
-  | _build
-  | buck-out
-  | build
-  | dist
 
-  # The following are specific to Black, you probably don't want those.
-  | blib2to3
-  | tests/data
-)/
+(
+  /(
+      \.eggs         # exclude a few common directories in the
+    | \.git          # root of the project
+    | \.hg
+    | \.mypy_cache
+    | \.tox
+    | \.venv
+    | _build
+    | buck-out
+    | build
+    | dist
+  )/
+  | foo.py           # also separately exclude a file named foo.py in
+                     # the root of the project
+)
 '''
-```
 
 </details>