Create a standard for Grey/Multivocal studies

[david-istvan]

Perhaps the best guideline for MLRs for SE are due to Garousi et al.: https://doi.org/10.1016/j.infsof.2018.09.006.

Suggested structure:

• The standard could elaborate on the Grey Review part of Multivocal Reviews in detail, and
• refer to the Systematic Reviews standard.

[drpaulralph]

We are working on breaking the systematic review standard into several more specific standards - for meta-analysis, meta-synthesis, case survey, etc. Once we know what that looks like, we can come back to how to incorporate multivocal literature.