Provide Kent's README.md for parser.

[KineticTheory]

Background

• @kgbudge has begun a series of "Why we did it that way" essays for the Capsaicin team, discussion reasons for architectural decisions.
• The first was for our parser code, which is now part of the Draco library. @KineticTheory suggested I add this to draco as a README.md file.
• There were some lint issues in Add a README.md file to src/parsers based on "Why we did it that way" essay #1076 that I fixed in this PR that replaces it.

Description of changes

• Add .mdl_style.rb to ignore some style warnings that we don't care about.
• Apologies to @kgbudge about Draco not being ready to accept his md file.

Status

• Reference:
  • Pre-Merge Code Review
  • CDash Status
• Checks
  • Travis/Appveyor CI checks pass
  • Code coverage does not decrease
  • Valgrind test passes
  • Toss3 checks pass
  • Trinitite checks pass
  • Code reviewed/approved, sufficient DbC checks, testing, documentation