You can find additional 1.13 Documents (Modding-Examples, Tools, ...) which are useful for 1.13 Modders in the following SVN Location: 
https://ja2svn.mooo.com/source/ja2/trunk/Documents/

1.13 Starter Documentation
https://github.com/aimnas/1.13_starter_documentation

How to get: latest 1.13, 7609, feature-descriptions and more
http://thepit.ja-galaxy-forum.com/index.php?t=msg&goto=361643&#msg_361643

Where to read about new game features:
--------------------------------------
Bears-Pit forum
http://thepit.ja-galaxy-forum.com/

Flugente's Magika Workshop
http://thepit.ja-galaxy-forum.com/index.php?t=thread&frm_id=283&

Trunk AI and improvements
http://thepit.ja-galaxy-forum.com/index.php?t=msg&th=24289

Where to post found bugs:
-------------------------
Bugs: Unofficial releases (SCI's), unstable and DEV builds
http://thepit.ja-galaxy-forum.com/index.php?t=msg&th=22776&start=0&

Official 1.13 svn links:
------------------------
Source: https://ja2svn.mooo.com/source/ja2/trunk/GameSource/ja2_v1.13
Gamedir: https://ja2svn.mooo.com/source/ja2_v1.13_data/GameDir
Language specific: https://ja2svn.mooo.com/source/ja2/trunk/GameData/