Add Hello World section

Mostly just pointing toward the in-progress hello_world.md doc.

Issue #162
stig_dm doc: In progress

Barely even bare bones at this point. Saving for posterity. Plus:
commit early, commit often.
Add stig_dm command/section

Issue #132

Just a first draft.
Start of hello_world.md

#162
Merge pull request #165 from vmbrasseur/master

Many doc changes