Add task to run black --check in ci & update README

.github/workflows/ci.yml

@@ -4,6 +4,24 @@ on: [push, pull_request]
 
 jobs:
 
+  check-formatting:
+    runs-on: ubuntu-latest
+
+    name: Consult black on python formatting
+
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions/setup-python@v1
+        with:
+          python-version: 3.7
+      - uses: dschep/install-pipenv-action@v1
+      - name: Install dependencies
+        run: |
+          pipenv install --dev --python ${pythonLocation}/python
+      - name: Run black
+        run: |
+          pipenv run black . --check --diff --exclude tests/output_
+
   run-tests:
     runs-on: ubuntu-latest
 

README.md

@@ -307,6 +307,18 @@ $ pipenv shell
 $ pip install -e .
 ```
 
+### Code style
+
+This project enforces [black](https://github.com/psf/black) python code formatting.
+
+Before commiting changes run:
+
+```bash
+pipenv run black .
+```
+
+To avoid merge conflicts later, non-black formatted python code will fail in CI.
+
 ### Tests
 
 There are two types of tests:
@@ -324,7 +336,7 @@ Adding a standard test case is easy.
 
 It will be picked up automatically when you run the tests.
 
-- See also: [Standard Tests Development Guide](betterproto/tests/README.md) 
+- See also: [Standard Tests Development Guide](betterproto/tests/README.md)
 
 #### Custom tests