Add docs section for converting from older builder files to v3

[samccann]

It was mentioned in a demo that this is fairly easy to do, but would be a common section readers might look for. iI'd suggest starting a small 'porting guide' for builder and add this to it.

From the demo, the steps are:

• add version 3 to the execution_environment.yml file
• if you refer to a builder image, remove it.
• if it is a version 1, you will need to add the images.

(take these steps w/ a grain of salt - needs to be verified with someone who knows.)