feat: add Samples section to CONTRIBUTING.rst (#181)

Source-Link: googleapis/synthtool@52e4e46
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:6186535cbdbf6b9fe61f00294929221d060634dae4a0795c1cefdbc995b2d605

document_ai/snippets/noxfile.py

@@ -28,8 +28,9 @@
 # WARNING - WARNING - WARNING - WARNING - WARNING
 # WARNING - WARNING - WARNING - WARNING - WARNING
 
-# Copy `noxfile_config.py` to your directory and modify it instead.
+BLACK_VERSION = "black==19.10b0"
 
+# Copy `noxfile_config.py` to your directory and modify it instead.
 
 # `TEST_CONFIG` dict is a configuration hook that allows users to
 # modify the test configurations. The values here should be in sync
@@ -159,7 +160,7 @@ def lint(session: nox.sessions.Session) -> None:
 
 @nox.session
 def blacken(session: nox.sessions.Session) -> None:
-    session.install("black")
+    session.install(BLACK_VERSION)
     python_files = [path for path in os.listdir(".") if path.endswith(".py")]
 
     session.run("black", *python_files)