Custom manifest.json and fixed some issues

[Dhinagaran-s]

#37 Miss content_type on manifest.json

Fixed with custom file option like serviceworker, by setting a path variable PWA_MANIFEST_PATH at the settings.py

#38 sizes not in html

Corrected the README.md file for the tutorial to use django-pwa

#59 Custom manifest.json

Fixed with custom file option like serviceworker, by setting a path variable PWA_MANIFEST_PATH at the settings.py

#70 Short Name Setting Not added

Created a variable to choose the SHORT NAME by using the variable PWA_APP_SHORT_NAME

[ali-0315]

hi
pls merge this , I need it! @silviolleite @hartungstenio