add WAI-ARIA guideline to CONTRIBUTING.md
add WCAG color guideline to CONTRIBUTING.md
Add ref in Less to docs regarding box-sizing conflicts

Refs #12351
Fix #12353: Scope table border reset in panels to first-child rows
Merge pull request #12355 from twbs/aria-contributing

add WAI-ARIA guideline to CONTRIBUTING.md
Merge pull request #12356 from twbs/wcag-contributing

add WCAG color guideline to CONTRIBUTING.md
Merge pull request #12357 from twbs/border-box-note

add note in Less source about global `box-sizing: border-box;`