Added instructions for downloading subtitles in README

[bandupahile]

🚨Please review the guidelines for contributing to this repository.

Proposed changes

I have added instructions in README for downloading subtitles during/after the course has been downloaded. This is in response to a suggestion by @rbrito in #524

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating
the PR. If you're unsure about any of them, don't hesitate to ask. We're here
to help! This is simply a reminder of what we are going to look for before
merging your code.

• [ x ] I have read the CONTRIBUTING doc
• [ x ] I agree to contribute my changes under the project's LICENSE
• I have checked that the unit tests pass locally with my changes
• I have checked the style of the new code (lint/pep).
• I have added tests that prove my fix is effective or that my feature works
• [ x ] I have added necessary documentation (if appropriate)

Further comments

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Reviewers

@rbrito

[coveralls]

Coverage remained the same at 47.7% when pulling 6e3b77f on bandupahile:master into 265718c on coursera-dl:master.

[coveralls]

Coverage remained the same at 47.7% when pulling 6e3b77f on bandupahile:master into 265718c on coursera-dl:master.