Update CONTRIBUTING.md

Author: jpenilla

CONTRIBUTING.md

@@ -192,6 +192,26 @@ and your server.
 While we will fix minor formatting issues, you should stick to the guide below
 when making and submitting changes.
 
+### Branches and Minecraft Versions
+
+Generally, PRs should be targeted at the `main` branch, where active development for
+the latest Minecraft release happens.
+
+If a new Minecraft release is imminent, it may be wise to wait until it's released
+and merged to `main` to avoid having to rebase your PR, as the `main` branch will be
+frozen during this time (you can ask in the Paper Discord if you're unsure).
+
+For old Minecraft versions - we are unlikely to accept PRs targeting any versions not
+marked as supported at https://fill-ui.papermc.io/projects/paper.
+
+For snapshots and other pre-release versions - you should only target these branches
+for changes specific to features or changes in those versions. For example, if Copper
+Golem is going to be added in 1.21.9, you can PR to the relevant dev branch as it would
+not make sense to merge that into `main`. Or, if you are fixing a bug that only exists in
+a dev branch, you should target that branch. When submitting PRs to dev branches, please
+coordinate with the dev team in Discord or open an issue before starting work to ensure
+it's an area we want to accept changes for.
+
 ## Formatting
 
 All modifications to Vanilla files should be marked. For historical reasons,