Fix docs URL in README.md

[anshsrtv]

The Readme states the following:

For full documentation about API endpoints use this URL:

http://127.0.0.1:8000/api/docs

There isn't any such page found. It should be replaced by

http://127.0.0.1:8000/api/schema/swagger-ui/

[sbs2001]

That is fixed via #379