Update example exclude to match only files in root (#1861)

* Update example exclude to match only files in root

The `exclude` section of the example `pyproject.toml` file didn't work
as expected. It claimed to exclude matched files only in the project
root, but it actually excluded matched files at any directory level
within the project. We can address this by prepending `^/` to the regex
to ensure that it only matches files in the project root.

See https://github.com/psf/black/issues/1473#issuecomment-740008873 for
explanation.

* Mention excluding directories as well

README.md

@@ -313,9 +313,10 @@ line-length = 88
 target-version = ['py37']
 include = '\.pyi?$'
 exclude = '''
-
-(
-  /(
+# A regex preceded with ^/ will apply only to files and directories
+# in the root of the project.
+^/(
+  (
       \.eggs         # exclude a few common directories in the
     | \.git          # root of the project
     | \.hg