Update install instructions

[selenaxiao-nist]

Committer Notes

Updates install instructions to include where oscal-cli executable is, how to install locally

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[iMichaela]

@selenaxiao-nist - I suggest keeping the guidance related to maven. We will need to update it with the latest sonatype changes, and only add (as oppose to replace) the local installation guidance.

[selenaxiao-nist]

@iMichaela I added the Maven guidance back.

[iMichaela]

@selenaxiao-nist - For non-developer, adding the oscal-cli to the path might be challenging without a step-by-step guidance (copy or soft link )

[selenaxiao-nist]

@iMichaela I added more to the adding to PATH section.