Adds installation of kramdown to README.

Jekyll uses the kramdown gem for markdown compilation. Without it the
project won't build.
Replace advice of using vim for Sublime Text.

This is considerably easier to use for non cli-savvy users.

Fixes #493
Include link to latest web-starter-kit release.
Update provide-real-time-validation.markdown

Changed IP Address validation into three different validations: IPv4, IPv6 and IPv4 + IPv6.
Merge pull request #508 from jbnicolai/add-krawmdown-to-readme

Adds installation of kramdown to README.
Adds Node and Grunt dependencies to README.
Adds Gemfile and Gemfile.lock and updates README.

Conflicts:
	README.md
Merge branch 'master' into adds-gemfile

Conflicts:
	src/README.md
Fixed spelling mistake
Merge pull request #517 from dowsieboi/spelling_mistake_fix

Fixed spelling mistake
Merge pull request #515 from jbnicolai/adds-gemfile

Adds Gemfile
Merge pull request #510 from jbnicolai/replace-vim-for-sublime

Replace vim for sublime
Merge pull request #509 from jbnicolai/link-latest-wsk-release

Include link to latest web-starter-kit release.
Fixes #497 - removes reference to cygwin
Merge branch 'master' of github.com:google/WebFundamentals
Merge pull request #511 from ikkebr/patch-1

Update provide-real-time-validation.markdown
Fixes #521 - updating the location of the images
Merge branch 'master' of github.com:google/WebFundamentals
Update provide-real-time-validation.markdown

fix typo
Added missing word