Update 09.1-animation.md

#### First PR and have completed the CLA request

The reason I chose to submit this is because it was initially unclear from the docs that the transition group needed to already be mounted in order for it to work properly.
I'll let @chenglou handle comments about content, but just a nit that this link will be broken in the website. It should be `/react/docs/animation.html#getting-started` or even more simply `#getting-started` since it's on the same page