Add a line mentioning that it can take up to 30 minutes for a newly published module to appear in the palette

[eamon0989]

After adding 2 custom nodes to flows.nodered.org, I went to the node-red palette but my nodes didn't appear there. I asked in the node-red slack and it seems that the catalog regenerates every 30 minutes. I'm guessing I am not the only person that was confused by this, it would be nice to add a line to the packaging.md#adding-to-flowsnoderedorg section mentioning that you may have to wait up to 30 minutes for it to appear in the search.

https://github.com/node-red/node-red.github.io/blob/master/docs/creating-nodes/packaging.md#adding-to-flowsnoderedorg