docs: Add USAGE.md

[geomars]

What does this PR do?

Close #32

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Add USAGE.md
• edit examples/example.py to be python 3 compatible

[SendGridDX]

All committers have signed the CLA.

[mbernier]

Why did you add this?

[geomars]

The original last line on examples/example.py is print header.json_string() instead of print(header.json_string()) . I thought it might be intentional not an error or typho to keep the python2 version than upgrade to python3 by just change to print(). That's why I add this line. The from__future__ import absolute_import means that if py2 user import string, python will look for upper-level string module, rather than current_package.string, this will prevent error in module import. from __future__ import absolute_import, print_function enable the scripts backward compatibility with python2 while moving to python3 version.

[mbernier]

NEAT!!

[mbernier]

This is great work - I am just confused about that future statement, as I am not sure it's necessary.

[thinkingserious]

Hello @geomars,

Thanks again for the PR!

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