Add integration with GitHub Actions CI/CD

[webknjaz]

Please configure integration with GitHub Actions CI/CD:

• Create a new branch called feature/14-gha-ci-cd-integration and do all the following steps within that branch.
• Add .github/workflows/ci.yml running pre-commit. Set it up to run Python 3.9 env. Ensure you install pre-commit before running tests:

name: tests

on:
  push:
  pull_request:

jobs:
  linters:
    runs-on: ubuntu-latest

    steps:
    - name: Fetch the src
      uses: actions/checkout@v2
    - name: Set up Python 3.9
      uses: actions/setup-python@v2
      with:
        python-version: 3.9

    - name: Install pre-commit
      run: python -m pip install --upgrade pre-commit

    - name: Run pre-commit
      run: python -m pre_commit run --all-files

• Add GitHub Actions CI/CD badge (master branch) to README.md with a single commit. ([![Build Status](https://github.com/kpi-web-guild/django-girls-blog-mike-011/workflows/tests/badge.svg)](https://github.com/kpi-web-guild/django-girls-blog-mike-011/actions?query=workflow%3Atests+branch%3Amaster))
• Push the branch and create a Pull Request for it.