Document the sync feature and environment variables

[moussetc]

Currently, there is a make sync command that is supposed to help keep plugins updated with the starter template, to benefit from latest improvements&updates done to the starter.

Sadly, it is not featured in the GH repository's README, nor easy of use (it fails for me, probably because I missed some pre-required steps which is part of the missing documentation).

There are also undocumented environment variables supported by the Makefile (like MM_DEBUG, MM_SERVICESETTINGS_ENABLEDEVELOPER...) that would be worth mentioning in the README, explaining how and why to use them would be interesting. I assume from the name that they are also used for Mattermost development, but not all plugin developers have knowledge of Mattermost development (nor a local MM dev environment, because you can use Docker to test a plugin).

[hanzei]

Does the documentation in https://github.com/mattermost/mattermost-plugin-starter-template/tree/master/build/sync cover your concerns about make sync?

👍 for properly documenting the env variables.

[moussetc]

Does the documentation in master/build/sync cover your concerns about make sync?

Not really, for 2 reasons:

1. The build/sync/README.md is about the generic syncing tool, not it's application on the specific mattermost-starter-template. For example it explains how to start the syncing using go run ... instead of make sync.
2. This README is buried in a sub-sub-folder, and no mention of the syncing feature is made on the root-level README, which it should be in my opinion (if the syncing feature is considered as mature enough, which it might not be)