Update README.md

README.md

 SSH and GPG public keys for Tétras Libre collaborators
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+# List GPG key : 
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+`gpg --list-keys`
+
+# Edit confidence a key : 
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+`gpg --edit-key <keyName>`
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+# Change confidence :
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+`trust`
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+Answer the response to add the confidence `5 - ultimate`. Use `quit` to leave.