Update cert-notice readme

[WithHolm]

PR Summary

Fix #2438

Updated so that there is a notice at top of readme and a list futher down of all certificate issuers, and thumbprint going back to 3.0.3

after installing all available versions from ps module store i ran the following command to list all the different module certificates

 get-module pester -ListAvailable|%{$sign = Get-AuthenticodeSignature -FilePath $_.Path;[pscustomobject]@{version=$_.version;issuer=$sign.SignerCertificate.Issuer;thumbprint=$sign.SignerCertificate.Thumbprint;subject=$sign.SignerCertificate.Subject}}|ft -a

after that i just manually found the different certificate thumbprint and figured out the certificate for the different certificate ranges.

I noted that 3.4.0 had a different certificate subject due to it being released as a w10 module, but this might be wrong.
3.0.3 -> 4.4.2 did not have any certificate (except for 3.4.0 )

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