Write a guide about how to add a new extension

[gbartolini]

Currently, the information in the BUILD.md is quite accurate, but it doesn't help a contributor who wants to add a new extension image and test it manually on their machines.

Let's reorganise the content in the BUILD.md by writing a new document that explains how to write a new extension image.

Link it from the README, too (the README might also need to be enhanced).