added gitignore to repo

[hanneshapke]

Description

Seems like the repo was missing a gitignore file since it was a copy from the internal git repo at Google.

related #241

Checklist

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• Read the How to Contribute guide
  if this is your first contribution.
• Open an issue to discuss this change.
• Update documentation if applicable.
• Write new tests if applicable.

[codesue]

Thanks for adding this! I'd prefer using GitHub's Python gitignore as the base since it's more commonly used and doesn't require attribution. From there, we can add:

# VSCode project settings
.vscode

# Bazel generated files
bazel-*
**/*_pb2.py
**/*_pb2_grpc.py

And change

*.py[cod]
*$py.class

# C extensions
*.so

to

**/*.py[cod]
**/*$py.class

# C extensions
**/*.so

This is similar to tfx/.gitignore but with more recent changes to GitHub's Python base incorporated.

[hanneshapke]

Thank you @codesue for your feedback. I updated the gitignore file.