Release process: move meta-warning earlier.

One needs to get the right doc from the start.

wiki/src/contribute/release_process.mdwn

@@ -4,6 +4,10 @@
 
 See the [[release_schedule]].
 
+<div class="caution">
+Read the remainder of this document from the branch used to prepare the release!
+</div>
+
 Requirements
 ============
 
@@ -77,10 +81,6 @@ Select the right branch
 What we refer to as the "release branch" (and `RELEASE_BRANCH`) should
 be `testing` for major releases, and `stable` for point-releases.
 
-<div class="caution">
-Read the remainder of this document from the branch used to prepare the release!
-</div>
-
 Sanity check
 ============