doc: add example using pin-source method

[111a5ab1]

Name of feature:

Add example of using pin-source method to main README.md

Pain or issue this feature alleviates:

Usage examples in the README.md all use pin-value, giving the impression the PIN must be passed in insecurely via command-line.

There is a single example usage of pin-source in key.go, but this is easily missed.

This pull request changes the first two usage examples to use the pin-source method, to better call out this more secure method of providing the pin.

Supporting links/other PRs/issues:

• README.md - General Usage
• key.go Line 42
• How to Handle Secrets on the Command Line
• Closes #94

[CLAassistant]

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