Robust .gitignore

[BoiseITGuru]

Description

Updated the .gitignore file to add standard extensions for each of the languages/platforms used in the examples. I also renamed flow.json to flow.json.example to follow security best practices and help prevent anyone from accidentally leaking any private keys in their own forks (even if it just local/testnet keys).

The readme has also been updated to note that flow.json.example needs to be copied/renamed.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

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