mark keyless pem files with b64 (#2671)

Signed-off-by: Batuhan Apaydın <batuhan.apaydin@trendyol.com>

README.md

@@ -783,3 +783,9 @@ We will also cut patch releases periodically as needed to address bugs.
 
 Should you discover any security issues, please refer to sigstore's [security
 process](https://github.com/sigstore/.github/blob/main/SECURITY.md)
+
+## PEM files in GitHub Release Assets
+
+The GitHub release assets for cosign contain a PEM file produced by [GoReleaser](https://github.com/sigstore/cosign/blob/ac999344eb381ae91455b0a9c5c267e747608d76/.goreleaser.yml#L166) while signing the cosign blob that is used to verify the integrity of the release binaries. This file is not used by cosign itself, but is provided for users who wish to verify the integrity of the release binaries. 
+
+By default, cosign output these PEM files in [base64 encoded format](https://github.com/sigstore/cosign/blob/main/doc/cosign_sign-blob.md#options), this approach might be good for air-gapped environments where the PEM file is stored in a file system. So, you should decode these PEM files before using them to verify the blobs.