add WAI-ARIA guideline to CONTRIBUTING.md
add WCAG color guideline to CONTRIBUTING.md
add ID to "Box sizing" section of Getting Started docs
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;`
Fix #12359: Reset min-width on fieldsets so that they don't break responsive tables and behave more like standard block level elements
Merge branch 'master' of github.com:twbs/bootstrap
Remove Twitter from tweet button
Merge pull request #12365 from Quy/master

Remove Twitter from tweet button
Clarify nesting in grid example