Update docs on how to add new releases

[hanzei]

Summary

generator add should be used to add releases for all not predefined plugins. This PR updated the documentation to make this clear.

Ticket Link

Fixes #74

[larkox]

LGTM. Only thing that comes to my mind is that mattermod automated message after a version cut ask to run "make plugins.json", instead cmd add. Should that also be changed?

[hanzei]

Good question @larkox. Given that sometime make plugins.json should be used and sometime generator add I'm not quite sure what mattermod should output. Maybe link this documentation?

[larkox]

It would break the flow for those that just want to copy paste those lines, but yes, link to this documentation may be needed. I wonder whether a small explanation on the message (for those that have done it over and over but fail to remember the syntax) on the two options would be nice too.

[jfrerich]

Thanks for adding the additional instructions! I added some suggestions for wording organization.

[hanzei]

@jfrerich @larkox After thinking a bit more about this PR, I don't consider supporting two ways of adding a release valuable any longer. Hence, I'm leaning toward deprecating make plugins.json in favour of using generate add. I've updated the doc to reflect that proposal. Let me know what you think.