DOCS: add UPDATING.md

This include the trainer's updating process, say goodbye to #2
references :P

UPDATING.md

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+# TRAINER UPDATE PROCESS
+
+Upgrade cycle when balance changes are released by NGE. Note that doing the
+change will require you to know how to operate a webserver to test, and doing
+some minor changes in the source code. While not complicated, you still need to
+know these basics.
+
+## Doing a balance update
+
+### Create a new version
+
+Copy the newest version folder in `data` to a new version number.
+
+### Change the trainer data
+
+Modify with the new `trainerdata.json` in that new folder with the new balance changes.
+
+### Change the icons
+
+Icons size is 47px width x 48px height. The ratio hasn't been kept to deal with
+some artifacts as there are different icon styles used by the game (square, rounded,
+with or without black border).
+
+**It's not recommended to re-edit the images from the older version**, due to the
+quality loss it implies. Lossless icons for all skills are available in the
+[CoRT Icons pack](https://github.com/mascaldotfr/CoRT/releases).
+
+That pack also includes a python script to automatically refresh skills icons from
+your edited `trainerdata.json`. Its usage is explained in the pack's README, and
+is the **heavily recommended method**.
+
+### Making the new version available on the website
+
+Update the array containing all available versions in `js/trainer.js` with the
+newest version last.
+
+## The version 66 problem
+
+I don't want to spread fear among people, because it won't happen, yet i can't
+left you ignorant of that fact.
+
+You can't have more than 66 datasets versions with the current short URL
+format. In the unlikely case we hit that limit, the simplest solution is to use
+an extra parameter, let's say `g` as generation and move the versions in a
+multidimensional array.
+
+In js/trainer.js:
+
+```javascript
+let trainerdataversions = [
+    [66 oldest versions, ...],
+    [x newer versions, ...]
+];
+```
+
+Then assume in `$(document).ready(...)` that an undefined `g` is
+`trainerdataserversions[0]`, otherwise it's `trainerdataserversions[g]`.
+
+On top of that you'll need to change `save_setup_to_url()` to add that
+parameter for future saved URLs.