Emojis are supported out of the box in Franklin. You can either copy paste emojis in your source code or use a shorthand such as :+1:: 👍.
The Emoji cheat sheet is a useful reference for emoji shorthand codes.
The dictionary for emojis that is used is Julia's emoji_symbols which may not contain all emojis (it should contain most though), you can check whether an emoji is present in your REPL with
using REPL.REPLCompletions: emoji_symbols
println(get(emoji_symbols, "\\:+1:", "not found"))
println(get(emoji_symbols, "\\:dragon_face:", "not found"))👍
🐲
N.B. Franklin supports Unicode Standard emoji characters, however the rendering of these glyphs depends on the browser and the platform. Make sure you select your font-stack appropriately.