Update security.txt with inline instructions for updating it

.well-known/security.txt

@@ -14,23 +14,45 @@ Policy: https://github.com/php/php-src/security/policy
 # supported version of PHP (at the time of the changes).
 # Supported PHP versions are listed at <https://www.php.net/supported-versions.php>.
 # Release manager PGP keys are listed at <https://www.php.net/gpg-keys.php>.
-
+#
+# To make changes to this file:
+#
+# 1. First, remove the PGP signature that wraps the body of this file:
+#
+#        gpg --decrypt --output security.txt security.txt
+#
+# 2. Make and save your changes to this file, i.e., update the Expires
+#    timestamp. Be sure to update the signature above to include your name,
+#    the email address for your PGP key, and the current date.
+#
+# 3. Sign your changes:
+#
+#        gpg --clearsign --local-user [email protected] --output security.txt.asc security.txt
+#
+#    Note: you cannot output the signature to the same file as the input or it
+#    will result in a signature wrapped around empty content.
+#
+# 4. Last, replace security.txt with security.txt.asc and commit your changes:
+#
+#        mv security.txt.asc security.txt
+#        git commit security.txt
+#
 # For more information about this file, see <https://securitytxt.org> and
 # RFC 9116 <https://www.rfc-editor.org/rfc/rfc9116>.
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