docs/fingerprint: Add note about generating keys
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TEST=view in gitile
@@ -187,6 +187,13 @@ devices.
Switching keys is only possible when the `RO` firmware is not write protected,
since the public portion of the keypair is stored in the `RO` firmware.
+### Generate Key
+
+For testing, you can generate a new key by using the following openssl command:
+```bash
+openssl genrsa -3 -out board/$BOARD/dev_key.pem 3072
+```
+
### Resources
* https://sites.google.com/a/google.com/chromeos/resources/engineering/releng/signer-documentation
