Add notes attribute to Artifact

[Nicholas_Bokulich]

great idea! Just linking to older discussion on the topic (with enclosed GH issue links):

Annotation/Comments option for artifacts

First off, big congrats on the release of qiime 2018.4 ! With the ever growing number of options and branches of the available pipelines I was thinking it might now be useful to have an annotation option when creating artifacts. For example: qiime feature-table rarefy \ --i-table table.qza --p-sampling-depth 2000 \ --o-rarefied-table rarefied-table.qza \ --comment "2000 depth drops S02 & S04 only, could increase to 7000 at a cost of losing S05 and S07 also" This wo…