From Civilization V Wiki
Below is a list of various guides to assist you in developing an amazing Civilization V mod.
Gameplay Database Overview - Learn how to create tables, as well as add, update, or delete rows in the database.
Localized Text Syntax - Learn the new localized text syntax for compositing complex yet grammatically correct sentences.
Adding a new written translation - Learn how to add a new written translation to the game.
River system overview - Understand the data structures and functions available to place a river on your map
Adding Replay Data Sets - Learn how to add extra datasets to be viewed in the replay graph.
Replay File Format - Get the technical details of the current replay file format.