Add to wiki, how to create wekan.spk from source, so it can be installed to Sandstorm instance and tested there.

I have not looked are there instructions somewhere already, so if there is, this can be closed with link to existing documentation.

From @ocdtrekkie

This should be relatively trivial to document. You pretty much just run spk pack in the source folder. (Though it might be meteor-spk pack actually, which would need to be installed, as well as Sandstorm itself.)

The important part is that a third party trying to build it will not have the required key to publish an app with the Wekan app ID. They would need to change it, which is a process documented here: https://docs.sandstorm.io/en/latest/developing/publishing-apps/#double-check-your-app-id

And they could not upgrade grains from official Wekan to a Wekan version they built themselves.

From @dwrensha

@ocdtrekkie Unfortunately it's not quite so simple.

Due to sandstorm-io/meteor-spk#25, a locally-modified version of meteor-spk v0.2.1 needs to be used (updated schemas need to be dropped in at meteor-spk.deps/node_modules/sandstorm/).

Due to the issue described here, I've been maintaining an upstream patch in Wekan that I apply before releasing.