Update README based on user feedback

Story

As a new user, I would like to have an updated readme, So that I can more easily get started without expert knowledge

Notes from user session:

• README is missing info on the trestle-config.yml file
• not clear what the difference is between the oscal types in the templates list and why one would care about them
• README missing step to copy component over from included templates
• README missing what in .trestle should be gitignored
• Better introduction as to what ssp-markdown and ssp-render are for
• Note how to include docs from other folders using md_clean_include
• Move the jinja hint to a new place as it was distracting
• More explanation on the “non-OSCAL SSP files” parts “frontmatter that is not part of the OSCAL schema
• mention that pandoc is an external command