Clarify README around using Awsaml vs. setting up an identity provider for Awsaml

[fmitchell-r7]

New users have expressed some confusion around running Awsaml. People think they need to set up an identity provider (like Okta) and configure their Amazon account before they can download the binary and run it. We should split the README out into separate documents that cover running Awsaml as a user vs. setting up an identity provider for use with Awsaml. This will also lay the groundwork for being able to provide documentation around identity providers beyond Okta.

Documentation should be Markdown formatted and go into a "docs" folder in the root. The following is probably a good starting point

• docs/usage.md - Covers how to get binaries and run Awsaml
• docs/setup.md - Covers how to get set up an identity provider

The README can then be amended with links to the individual documents.