Clarify the environments used for example development

[jbusecke]

Maybe mention that even if these are 2 different git repositories, they use the same conda environment, which instructions are found in the xgcm contributors guide.
That was this point that was not clear for me.

Originally posted by @rcaneill in #12 (comment)

[hdrake]

It seems like this repo must use a different environment because it uses packages like intake.