Create README.md

Author: AlexanderMelde

README.md

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+# YAPF-Action
+A GitHub action that runs [YAPF](https://github.com/google/yapf) to test if your python code is correctly formatted.
+
+The action uses the `--diff` parameter of YAPF to return
+- SUCCESS: exit-code=zero → no changes were necessary, code is YAPF-formatted
+- FAIL: exit-code=non-zero → not correctly formatted or program error
+
+## Usage
+To use this action in your repository, create a file like `.github/workflows/yapf_check.yml` with the following content:
+
+```
+name: YAPF Formatting Check
+on: [push]
+jobs:
+
+build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+    - name: run YAPF to test if python code is correctly formatted
+      uses: AlexanderMelde/yapf-action@master
+```
+
+You can fork and modify the `entrypoint.sh` script to use other [yapf parameters](https://github.com/google/yapf#usage), e.g. for setting a different code style (default is PEP8).