Clarify animating one or zero items in docs

When I first read these docs, it was not immediately clear to me that I could
use `React.addons.CSSTransitionGroup` to animate a single item coming into view,
or an item replacing an item already there. This was partly due to the example
which rendered a list of items.

This PR adds a blurb about being able to use
`React.addons.CSSTransitionGroup` with one or zero items, provides a code
example, and adds a note blockquote that a `key` attribute must always be
provided on each child of `React.addons.CSSTransitionGroup`. The latter point
was not immediately obvious from the original `TodoList` code example, since it
renders a list which would normally require `key` attributes anyway.

Test Plan:
- Refreshed `http://localhost:4000/react/docs/animation.html`, saw that the
  docs additions rendered correctly.
- Example code not tested (it was extracted from working code).
Community Round-up #20
Merge pull request #1942 from LoukaN/roundup_20

Community Round-up #20
Wording tweaks to community roundup
Fix typo for 'fundations'
Merge pull request #1943 from shaneosullivan/patch-1

Fix typo for 'fundations'
Fix small language nits
Update 2014-07-28-community-roundup-20.md
Merge pull request #1941 from divad12/animating-one-or-zero-items-docs

Clarify animating one or zero items in animation docs
Merge pull request #1944 from shaneosullivan/patch-3

Fix small language nits
Rename "Ice Lab studio" to "Icelab"
Adding "Tutorial" label to Flux Todo List nav item
Updating "Tutorial" label as per suggestion
Merge pull request #1951 from JedWatson/patch-1

Adding "Tutorial" label to Flux Todo List nav item
cssFloat is only accessible as styleFloat on IE8
Merge pull request #1854 from syranide/floatfix

cssFloat is only accessible as styleFloat on IE8