Advertise the official emscripten image

[trzecieu]

Thanks to @Squareys we have some movement in the creation of the official docker image for emscripten, hence I can start to think what's the place of this image and how to advertise using the official docker image.

Related:

• Build docker image on Circle CI emscripten-core/emsdk#513
• Followup to #368 - Emscripten Docker Image  emscripten-core/emsdk#372

CC:
@kripken
@Squareys