Add documentation on how to do releases

[no changelog]

docs/How to release.md

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+<!--
+SPDX-FileCopyrightText: 2023 Linnea Gräf <nea@nea.moe>
+
+SPDX-License-Identifier: CC0-1.0
+-->
+
+# How to create a release
+
+- Bump the version on gradle.properties
+- Create a tag with that same version (without `v` prefix, please)
+- Create a changelog based on
+  `git log --pretty=' - %s' --grep '[no changelog]' --invert-grep --fixed-strings oldversion..newversion | tac`, while
+  filtering out commits that should not be in the changelog.
+- Upload to [GitHub](https://github.com/romangraef/Firmament/releases/new)
+- Upload to [Modrinth](https://modrinth.com/mod/firmament/versions)
+- Send a message in [Discord](https://discord.com/channels/1088154030628417616/1108565050693783683)
+- Send a message in [the thread](https://hypixel.net/threads/firmament-a-skyblock-mod-for-1-20-1-fabric.5446366/)