Add note to Gatsby migration guide about using `next/image` with custom loaders. (#28805)
Add note to Gatsby migration guide about using `next/image` with custom loaders. (#28805)

Feedback from this article: https://blog.whereisthemouse.com/image-optimization-for-static-nextjs-sites
zeit/next.js,flybayer/next.js,flybayer/next.js,azukaru/next.js,azukaru/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,flybayer/next.js,flybayer/next.js,zeit/next.js,JeromeFitz/next.js,zeit/next.js,JeromeFitz/next.js,azukaru/next.js