Documentation for fix for failed testing during PRs from forks

[geoffwoollard]

Propose documentation for a workaround in https://github.com/compSPI/ioSPI/pull/62

Tested the steps of this work around.

• https://github.com/compSPI/ioSPI/pull/61

• https://github.com/compSPI/ioSPI/pull/60

• https://github.com/compSPI/ioSPI/pull/54

An example of this error coming up is in https://github.com/compSPI/ioSPI/pull/52, which is a PR from the fork luis-sribeiro.

[geoffwoollard]

There's a similar issue in Docker Hub credentials. See https://github.com/compSPI/simSPI/pull/67#issuecomment-1036886405