RELEASE-PROCEDURE.md: explain release candidates

Closes #16622

docs/RELEASE-PROCEDURE.md

@@ -94,6 +94,28 @@ of common public holidays or when the lead release manager is unavailable, the
 release date can be moved forwards or backwards a full week. This is then
 advertised well in advance.
 
+Release Candidates
+------------------
+
+We ship release candidate tarballs on three occasions in preparation for the
+pending release:
+
+- Release candidate one (**rc1**) ships the same Saturday the feature freeze
+  starts. Twenty-five days before the release.
+
+- Release candidate two (**rc2**) ships nine days later, sixteen days before
+  the release. On a Monday.
+
+- Release candidate tree (**rc3**) ships nine days later, seven days before
+  the release. On a Wednesday.
+
+Release candidate tarballs are ephemeral and each such tarball is only kept
+around for a few weeks. They are provided on their dedicated webpage at:
+https://curl.se/rc/
+
+**Do not use release candidates in production**. They are work in progress.
+Use them for testing and verification only. Use actual releases in production.
+
 Critical problems
 -----------------