Update README.md.

Mention that Python needs to be in PATH for Pygments to work and thus jekyll not fail.
README.md: mention an alternative workaround for the jekyll issue on Windows.
Help Windows users install Jekyll
Merge pull request #11832 from twbs/run_jekyll_on_windows

Help Windows users install Jekyll
Merge pull request #11830 from XhmikosR/jekyll-windows

Update Jekyll on Windows info
Combine updates to Windows Jekyll stuff
Clarify Windows workarounds.
customizer.js: remove a workaround for older less.js.
Merge pull request #11838 from XhmikosR/customizer

customizer.js: Remove a workaround for older less.js.
Use variables for breadcrumb padding
Merge pull request #11839 from XhmikosR/readme

Clarify Windows workarounds.
Merge pull request #11841 from ZDroid/breadcrumbs

Use variables for breadcrumb padding
Fixes #11837: grid containers are fixed width now, no longer max-width
Merge branch 'master' of github.com:twbs/bootstrap
Add @dropdown-caret-color back to variables.less
Added back @dropdown-caret-color back for .dropup
Removed @dropdown-caret-color from dropdowns.less
Added deprecated comment for @dropdown-caret-color
Merge branch 'master' of https://github.com/dzwillia/bootstrap