docs: autoupdater: document manifest header

Explains the behaviour when DATE is either in the future or in the past
and hints at how the firmware rollout can be controlled using the
PRIORITY variable.

Co-Authored-By: Martin Weinelt <martin@darmstadt.freifunk.net>

docs/features/autoupdater.rst

@@ -30,6 +30,27 @@ in ``site.mk``, care must be taken to pass the same ``GLUON_RELEASE`` to ``make
 as otherwise the generated manifest will be incomplete.
 
 
+Manifest format
+------------------------
+
+The manifest starts with a short header, followed by the list of firmwares and signatures.
+The header contains the following information:
+
+.. code-block:: sh
+
+    BRANCH=stable
+    DATE=2020-10-07 00:00:00+02:00
+    PRIORITY=7
+
+- ``BRANCH`` is the autoupdater branch name that needs to match the nodes configuration.
+- ``DATE`` specifies when the time period for the update begins. Nodes will do their regular update during a random minute
+  between 4:00 and 4:59 am. Nodes might not always have a reliable NTP synchronization, which is why a fallback mechanism
+  exists, that checks for an update, and will execute if ``DATE`` is at least 24h in the past.
+- ``PRIORITY`` can be configured as ``GLUON_PRIORITY`` when generating the manifest or in ``site.mk``, and defines
+  the number of days over which the update should be stretched out after ``DATE``. Nodes will calculate a probability
+  based on the time left to determine when to update.
+
+
 Automated nightly builds
 ------------------------