external-outs: mention `dvc add url`

Currently, we only mention that you can specify your remote output in the dvc run command, but we don’t say anything about an ability to dvc add your remote files.

[AJ-54]

I have made a pull request by performing the required changes as understood by me. Kindly have a look @efiop and suggest changes if required. Thank you for the coordination!!

[efiop]

@AJ-54 For example

    # Add S3 remote to be uses as cache location for S3 files
    $ dvc remote add s3cache s3://mybucket/cache

    # Tell dvc to use the 's3cache' remote as S3 cache location
    $ dvc config cache.s3 s3cache

    # Add data on S3 directly
    $ dvc add s3://mybucket/mydata

    # Run the stage with external S3 output
    $ dvc run \
              -d data.txt \
              -o s3://mybucket/data.txt \
              aws s3 cp data.txt s3://mybucket/data.txt