Add shrink-to-fit to getting started

See https://github.com/twbs/bootstrap/pull/17793#issuecomment-146960473

Add Glyphicons migration guidance
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Add Wall of Browser Bugs entry for #17438 / #14237

https://code.google.com/p/chromium/issues/detail?id=534750
Closes #17438
Closes #14237
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Reunites the family.
Closes #17791

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IE 9 not supported list - flexbox

add flexbox to the not supported list 

https://github.com/twbs/bootstrap/issues/17802

Add shrink-to-fit viewport directive (fixes potential iOS9 issues)

Fixes potential problems with iOS9's new viewport behavior - see for
instance https://forums.developer.apple.com/thread/13510

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Add SCSS configuration for Hound

This change uses Bootstrap's existing `scss/.scss-lint.yml` file
to configure Hound's hosted SCSS-Lint instance.

On each pull request to Bootstrap,
Hound will comment on any SCSS style violations in-line, like this:

![screenshot](https://images.thoughtbot.com/hound/scss-example.png)

If you update the pull request to adopt a suggestion,
the comment will be hidden.

It leaves the existing linting done by Grunt + Travis.

Hound is free for open source projects
and is open source itself:

https://github.com/thoughtbot/hound

Fix #17612: Throw explicit error when a nonexistent method is invoked

Remove http://bugzil.la/1048279 entry from Wall of Browser Bugs
https://bugzilla.mozilla.org/show_bug.cgi?id=1048279 has been closed as WONTFIX.
Refs #14124.
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Update jspm package configuration

Make the tooltip-viewport example work

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Remove random trailing less-than symbol

Rename .scsslint.yml to .scss-lint.yml

v4-dev - Clarify the purpose of CSS class .in to .fade
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Use backticks/<code> for JS snippets in docs headings

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Add <h3> header example to Cards docs

Change grid and container sizes to px

Filled leftover placeholder in docs

Changes grid and container sizes to `px`, as the
viewport pixel size does not depend on the font size.

The actual em values were inconsistent with the docs,
while the docs were not the same as the comments:

* `sm` breakpoint was 34em (544px) not 480px.
* `lg` container max-width was 60rem (960px), less gutter than `md`.
  Changed to 940px, same as Bootstrap 3.
* `xl` container max-width was 72.25rem which is 1140px not 1156px.
  Changed to 1140px matching the comment but not the docs.

Addresses #17070 and #17388.

Port #17712 over to Bootstrap v4

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