docs: document the staging branch and add reference to it

A while back we agreed that having a live/staging branch is beneficial.
Sadly we forgot to document that, so here is my first attempt.

Document the caveat that the branch history is not stable.

CC: Andres Gomez <agomez@igalia.com>
CC: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>

docs/releasing.html

@@ -21,6 +21,7 @@
 <li><a href="#overview">Overview</a>
 <li><a href="#schedule">Release schedule</a>
 <li><a href="#pickntest">Cherry-pick and test</a>
+<li><a href="#stagingbranch">Staging branch</a>
 <li><a href="#branch">Making a branchpoint</a>
 <li><a href="#prerelease">Pre-release announcement</a>
 <li><a href="#release">Making a new release</a>
@@ -212,6 +213,25 @@ system and making some every day's use until the release may be a good
 idea too.
 </p>
 
+<h1 id="stagingbranch">Staging branch</h1>
+
+<p>
+A live branch, which contains the currently merge/rejected patches is available
+in the main repository under <code>staging/X.Y</code>. For example:
+</p>
+<pre>
+	staging/18.1 - WIP branch for the 18.1 series
+	staging/18.2 - WIP branch for the 18.2 series
+</pre>
+
+<p>
+Notes:
+</p>
+<ul>
+<li>People are encouraged to test the staging branch and report regressions.</li>
+<li>The branch history is not stable and it <strong>will</strong> be rebased,</li>
+</ul>
+
 
 <h1 id="branch">Making a branchpoint</h1>
 

docs/submittingpatches.html

@@ -253,6 +253,9 @@ If you are not the author of the original patch, please Cc: them in your
 nomination request.
 </p>
 
+<p>
+The current patch status can be observed in the <a href="releasing.html#stagingbranch">staging branch</a>.
+</p>
 
 <h3 id="thetag">The stable tag</h3>