Add MC 26.1 to Upgrade Guide

Author: PseudoKnight

src/main/resources/docs/Upgrade_Guide

@@ -3,15 +3,24 @@ Below is a reverse chronological guide to help you with the most notable of thes
 Where possible, any value changes are converted for backwards compatibility with saved data, but any script that checks output for equality will need to be updated.
 ----
 
+== Minecraft 26.1 ==
+* The world file structure has changed. This only affects scripts directly reading or writing these files.
+
+----
+
 == Minecraft 1.21.11 ==
 * GameRule names have been changed and are now snake_case. While old names are converted at runtime, this is not possible when using the array from get_gamerule(world)
 * Old GameRules starting with "disable" have been inverted. These are inverted at runtime when using the old names.
 * Old GameRules "doFireTick" and "allowFireTicksAwayFromPlayer" have been replaced with "fire_spread_radius_around_player".
-* Several GameRules have new integer limits. (these appear to reset the next time the world attempts to save, but may case exceptions in the future)
+* Several GameRules have new integer limits. (these appear to reset the next time the world attempts to save, but may cause exceptions in the future)
+
+----
 
 == Minecraft 1.21.9 ==
 * CHAIN renamed to IRON_CHAIN
 
+----
+
 == Minecraft 1.21.5 ==
 * SPLASH_POTION entity type has been split into LINGERING_POTION and SPLASH_POTION again.
 * Some vanilla keys passed to the tellraw functions and commands have changed.