ddh_intro.md: market to interested users.

Believing this page to be targetted to encouraging people to contribute:

- Explain how instant answers have benefited them as users.
- Make it clear that everyone, regardless of skill level, can provide
  valuable feedback.
- Show that the feedback actually leads to a better experience for everyone.
- Ask them to help.
- Create a consistent hierarchy of changes (better sources -> more
  answers -> coding on answers)
ddh_intro.md: address feedback to rewrite.

Incorporating many excellent suggestions from @jagtalon while still
trying to maintain this structure.
ddh-intro.md: apply more recommended changes.

- Incorporate improved verbiage from @moollaza ad @jagtalon.
- Rework Quixley paragraph to better explain how it relates.
- Update some wording and ordering to keep everything aligned with
  the changes.
- Eliminate a redundant and repetitive line which made itself clear.
ddh-intro.md: fix Quixey typo.
ddh_intro.md: revert Quixey example sentence.
Merge pull request #30 from mwmiller/intro

ddh_intro.md: market to interested users.