Provide documentation how to upload new releases to the update site

Requested by Curtis Rueden because he does not want me to feel like
I am special, being the only one who knows how to do it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'releases'

Adds information how to upload releases to the update site.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>