Add labels explanation to readme
Fix Issue #15076.

This commit adds documentation explaining the need to have an initial
.active element in carousels.
Improved accessible glyphicon advice/use

...to actually make it consistent with what I said in the glyphicon docs
section...
aria-hidden added to some stray glyphicons

plus minor tweak to customizer's generated alerts
Merge pull request #15085 from patrickhlauke/glyphicons-in-forms-tweak

Glyphicon tweaks
Fixes #14939: Don't remove focus outline from buttons in button groups
Merge pull request #15077 from jayanthkoushik/issue-15076-fix

Fix Issue #15076.
Merge pull request #15086 from twbs/btn_group_focus

Don't remove focus outline from buttons in button groups
Wash-up of some stray dropdown examples

adding explicit aria-expanded="false" initial state (so they're
correctly announced by AT as being collapsed), and - in the case of
old-school use of <a> - role="button"
Merge pull request #15087 from patrickhlauke/dropdown-accessibility-tweaks

Wash-up of some stray dropdown examples
Dropdown: ignore keydown events coming from `input`s and `textarea`s

Fixes #15084.
Merge pull request #15088 from twbs/dropdown-keydown-ignores

Dropdown: ignore keydown events coming from `input`s and `textarea`s
Merge branch 'master' into labels-readme
move to the contributing guidelines
move single-toggle button plugin callout before example for consistency
move carousel accessibility callout next to rest of callouts
Merge pull request #14900 from twbs/labels-readme

Add labels explanation to readme
move data attributes callout into data attributes doc subsection
move popover examples heading to directly before the actual examples
move scrollspy callouts so they're all adjacent to each other