message: readme update about env vars
diff: This is
  - [x] Auth
  -- [x] S3-compatible Object Storage
  -- [ ] Server-Side Functions
  +- [x] Files (S3-compatible Object Storage)
  for Hasura
  -** This repo is under heavy development right now! :D **
  -
  ## Deploy
  -Add to docker-compose.yaml
  +Add to `docker-compose.yaml`:
  ```
  hasura-backend-plus:
  image: elitan/hasura-backend-plus
  environment:
  USER_FIELDS: '<user_fields>'
  - GRAPHQL_ENDPOINT: https://<hasura-graphql-endpoint>
  - HASURA_ACCESS_KEY: <hasura-access-key>
  - JWT_SECRET: <jwt secret>
  + HASURA_GRAPHQL_ENDPOINT: https://<hasura-graphql-endpoint>
  + HASURA_GRAPHQL_ACCESS_KEY: <hasura-access-key>
  + HASURA_GRAPHQL_JWT_SECRET: '{"type": "HS256", "key": "secret_key"}'
  S3_ACCESS_KEY_ID: <access>
  S3_SECRET_ACCESS_KEY: <secret>
  S3_ENDPOINT: <endpoint>
  @@ -52,9 +49,9 @@ Restart your docker containers
  ENV VARIABLES:
  ```
  USER_FIELDS: '<user_fields>'
  -GRAPHQL_ENDPOINT: https://<hasura-graphql-endpoint>
  -HASURA_ACCESS_KEY: <hasura-access-key>
  -JWT_SECRET: <jwt secret>
  +HASURA_GRAPHQL_ENDPOINT: https://<hasura-graphql-endpoint>
  +HASURA_GRAPHQL_ACCESS_KEY: <hasura-access-key>
  +HASURA_GRAPHQL_JWT_SECRET: '{"type": "HS256", "key": "secret_key"}'
  S3_ACCESS_KEY_ID: <access>
  S3_SECRET_ACCESS_KEY: <secret>
  S3_ENDPOINT: <endpoint>
  