Improve PGP key instructions in release guide

Author: HeartLinked

.github/workflows/rc.yml

@@ -49,10 +49,10 @@ jobs:
 
       - name: Archive
         run: |
-          id="apache-iceberg-cpp-${VERSION}-rc${RC}"
-          tar_gz="${id}.tar.gz"
+          ARCHIVE_FILE_NAME="apache-iceberg-cpp-${VERSION}-rc${RC}"
+          tar_gz="${ARCHIVE_FILE_NAME}.tar.gz"
           echo "TAR_GZ=${tar_gz}" >> ${GITHUB_ENV}
-          git archive HEAD --prefix "${id}/" --output "${tar_gz}"
+          git archive HEAD --prefix "${ARCHIVE_FILE_NAME}/" --output "${tar_gz}"
           sha512sum "${tar_gz}" > "${tar_gz}.sha512"
 
       - name: Audit

dev/release/README.md

@@ -53,20 +53,27 @@ Requirements to run `release_rc.sh`:
 If you don't have a PGP key, https://infra.apache.org/release-signing.html#generate
 may be helpful.
 
-Your PGP key must be registered to the following:
+Your PGP key must be published in the KEYS file, which is hosted at:
 
     * https://downloads.apache.org/iceberg/KEYS
 
 See the header comment of them for how to add a PGP key.
 
-Apache Iceberg committers can update them by Subversion client with their ASF account.
-e.g.:
+If you are a first-time release manager, you need to add your public key to this file. To prevent formatting errors, please use the following commands instead of editing the file manually:
+
++ Check out the release distribution directory:
 
 ```console
 $ svn co https://dist.apache.org/repos/dist/release/iceberg
 $ cd iceberg
-$ editor KEYS
-$ svn ci KEYS
+```
++ Append your GPG public key to the KEYS file. Replace <YOUR_KEY_ID> with your actual GPG key ID.
+
+```console
+$ echo "" >> KEYS   # append a newline
+$ gpg --list-sigs <YOUR_KEY_ID> >> KEYS  # append signatures
+$ gpg --armor --export <YOUR_KEY_ID> >> KEYS # append public key block
+$ svn commit -m "Add GPG key for <YOUR_NAME>"
 ```
 
 ### Publish