Update README.md

 ## Building
 
-# Python Version
+### Python Version
 
     ~$ python3 --version
     Python 3.8.2
 
 Python 가상환경 사용을 권장합니다.
 
-# pip, django
+### pip, django
 
     ~$ python3 -m pip install --upgrade pip
     ~$ pip install django~=3.1.2
     ...
     Successfully installed asgiref-3.2.10 django-3.1.2 pytz-2020.1 sqlparse-0.3.1
     
-# Database
+### Database
 
     ~/smartdoorlock-backend$ python manage.py migrate
     ~/smartdoorlock-backend$ python manage.py makemigrations
     
-# Run Server
+### Run Server
 
     ~/smartdoorlock-backend$ python manage.py runserver