Add labels explanation to readme
Accessibility tweaks and explanation for glyphicons

- add aria-hidden="true" to avoid SRs unintentionally reading out the
Unicode character (see
http://www.filamentgroup.com/lab/bulletproof_icon_fonts.html)
- callout with advice on accessible icon usage
- replaced sr-only text in examples with (in the case of button) cleaner
aria-label
- additional example of icon used to convey meaning (in a
non-interactive control)
Meaningful icons in about page
Add aria-hidden to icons in about page
Merge pull request #15009 from patrickhlauke/glyphicon-accessibility-tweaks

Glyphicon accessibility tweaks
Update devDependencies.
Run `grunt`.
HTML validation: No longer relax the `X-UA-Compatible` error

The validator no longer complains about it unless the value is different from `content=IE=edge`, see https://www.w3.org/Bugs/Public/show_bug.cgi?id=27091
Merge pull request #15072 from twbs/html-validator-x-ua-compatible

HTML validation: No longer relax the `X-UA-Compatible` error
Remove X-UA-Compatible caveat from Validators docs

Refs #15072
Merge pull request #15073 from twbs/15072-more

Remove X-UA-Compatible caveat from Validators docs
Assorted accessibility (and some consistency) fixes for documentation

Closes #14951 by merging it.
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"