docs(gatsby-image): Add SEO advice to the readme

[fsgreco]

Description

I added a new advice under the "Other stuff to be aware" section.
During my report on github I have been invited (by @freiksenet ) to point out this issue on the documentation.

Related Issues

You can find more details and the discussion here: #13742

[wardpeet]

Thank you for adding this to our docs. It's a good addition as not everyone knows about the problems this can cause.

[wardpeet]

Thanks for updating!

[gatsbot]

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

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