devguide: add an upgrade section

Add an upgrade section to the devguide. This should cover any changes
to APIs that users might be using from plugins or as a library user.

doc/userguide/devguide/index.rst

@@ -9,3 +9,4 @@ Suricata Developer Guide
    internals/index.rst
    extending/index.rst
    libsuricata/index.rst
+   upgrading/index.rst

doc/userguide/devguide/upgrading/index.rst

@@ -0,0 +1,21 @@
+Upgrading
+=========
+
+Upgrading 7.0 to 8.0
+--------------------
+
+EVE File Types
+~~~~~~~~~~~~~~
+
+- The ``ThreadInit`` function will now be called when in *threaded*
+  and *non-threaded* modes. This simplifies the initialization for EVE
+  filetypes as they can use the same flow of execution for both
+  modes. To upgrade, either remove the call to ``ThreadInit`` from
+  ``Init``, or move per-thread setup code from ``Init`` to
+  ``ThreadInit``.
+- Many of the function arguments to the callbacks have been made
+  ``const`` where it made sense.
+
+Please see the latest example EVE filetype plugin for an up to date
+example.
+