Update README.md

As per the discussion here https://github.com/jejacks0n/mercury/issues/304
give iframe a name
Add a test to the modal spec to ensure that the .mercury-modal and .mercury-modal-overlay elements get reused by all modals opened after the first one.
Reuse .mercury-modal and .mercury-modal-overlay divs instead of creating new ones every time a modal is opened.
Merge pull request #337 from nreckart/pr/reuse-modal-divs

Reuse .mercury-modal and .mercury-modal-overlay elements
spec asserting iframe id/name and class are the same
Merge pull request #336 from JonRowe/give_iframe_a_name

Give the <IFRAME> a name attribute
fixes the mocking of console.log.
prevent infinite-loops when loading mercury editor
adds developer note.
fixes grammar.
Merge pull request #352 from trongrg/fix_infinite_loops

prevent infinite-loops when loading mercury editor
fix 'Error: uninitialized constant Mercury::Engine' when running 'rails g mercury:install:authentication'
fix "Error: uninitialized constant Mercury::Engine" when running "rails g mercury:install:authentication" in a Rails Engine
Merge pull request #358 from cantonic/master

Fix "Error: uninitialized constant Mercury::Engine" when running the install generator in a Rails Engine
Fixed inserting Snippet in Chrome bug (issue #362)
Converts match routes to get routes for Rails 4.
Merge pull request #364 from simonharrer/replace-match-with-get

Converts match routes to get routes for Rails 4.
Change dependencies to be rails4 compatible
Merge pull request #366 from simonharrer/change-dependencies-to-be-rails4-compatible

Change dependencies to be rails4 compatible