Add a note about gif compatibility

[evadecker]

Gifs don't play well with gatsby-image for now, so we want to let people know a workaround. This adds a note to the bottom of the doc explaining what to do.

Based on a Twitter discussion: https://twitter.com/gatsbyjs/status/1050915984175853569

[DSchau]

Hey, thanks! Sorry you ran into this issue, but we really appreciate you taking the time to open an issue and rectify this. 💪

[gatsbot]

Holy buckets, @kybradeck — we just merged your PR to Gatsby! 💪💜

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