remind users to subscribe to a paid plan for the github org in question

[maxheld83]

I just successfully confused myself by signing up for flakehub with my personal GH account, and then expecting the flakehub cache to work for a repo inside a GH org I own.

I needed to also subscribe to flakehub for the org.

Might be nice to remind users about this, in case other folk are as dense as I am 😅.

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BTW, happy to add a PR for this (and other small fry) but I am majorly confused as to where the DetSys docs live on GH:

• this repo is (as per the last commit to main) out of date with what the footer says on https://docs.determinate.systems/flakehub/
• the gh link on https://docs.determinate.systems is broken, as per GitHub link in header 404s #19

[imgrant]

I remain confused, I also signed up to FlakeHub with my personal GitHub account, I just wanted to publish a public flake but the GitHub Action fails with "Organization imgrant not found."
I tried adding the FlakeHub GitHub app, but I get an error message that I tried to use my personal account which is a no-no.
It's not clear from the docs, whether for an individual, one must still have a GitHub organisation.