README one-line: add link to cached auth settings example file

[bbrewington]

What does this PR do?

Minor change to README.md - adds link to the example.py file showing users how to cache (same line is actually included in the docs/usage.rst file)

Why was this PR needed?

Quicker reference for people working out of the github repo rather than readthedocs.io

What are the relevant issue numbers?

did not create issue, since this is a one-line change just to README

Does this PR meet the acceptance criteria?

(I'm marking all 4 as pass given the above note in issue numbers question)

• [X] Passes flake8 (refer to .travis.yml)
• [X] Docs are buildable
• [X] Branch has no merge conflicts with master
• [X] Is covered by a test