docs: add migration guide

Includes tag-merge workflow from @toast 's post at
https://mk.toast.cafe/notes/93o9n4tfsi

docs/migrating.md

@@ -0,0 +1,59 @@
+# Migrating to FoundKey
+
+Migrating from Misskey to FoundKey is relatively straightforward. However, additional steps are required as there are significant changes between the two projects.
+
+## Backup
+The process will take some time and it's possible something will go wrong. It's highly suggested to make a database dump using `pgdump` and backing up `.config/default.yml` and the `files/` directory before proceeding any further.
+
+## Reverting migrations
+If you're migrating from Misskey 12.112.0 or higher, you'll need to revert some database migrations as they have diverged from that point. Specifically, you'll need to revert `nsfwDetection1655368940105` and newer migrations.
+
+Run the following to revert those migrations:
+```sh
+cd packages/backend
+
+LINE_NUM="$(npx typeorm migration:show -d ormconfig.js | grep -n nsfwDetection1655368940105 | cut -d ':' -f 1)"
+NUM_MIGRATIONS="$(npx typeorm migration:show -d ormconfig.js | tail -n+"$LINE_NUM" | grep '\[X\]' | nl)"
+
+for i in $(seq 1 $NUM_MIGRAIONS); do
+	npx typeorm migration:revert -d ormconfig.js
+done
+```
+
+## Switching repositories
+To switch to the FoundKey repository, do the following in your Misskey install location:
+```sh
+git remote set-url origin https://akkoma.dev/FoundKeyGang/FoundKey.git
+git fetch origin
+```
+We recommend using a local branch and merging in upstream releases as they get tagged. This allows for easy local customization of your install.
+
+For example, say your local branch is `toast.cafe` and you want to use release `v13.0.0-preview1`. To create that branch:
+```sh
+git checkout tags/v13.0.0-preview1 -b toast.cafe
+```
+
+When a new release comes out, simply fetch and merge in the new tag. Here we opt to squash upstream commits as it allows for easy reverts in case something goes wrong.
+```sh
+git fetch -t
+git merge tags/v13.0.0-preview2 --squash
+  # you are now on the "next" release
+```
+
+## Rebuilding and running database migrations
+This will be pretty much the same as a regular update of Misskey. Note that `yarn install` may take a while since dependency versions have been updated or removed and we use a newer version of Yarn.
+```sh
+yarn install
+NODE_ENV=production yarn build
+yarn migrate
+```
+
+## Restarting your instance
+To let the changes take effect restart your instance as usual:
+```sh
+# Systemd
+systemctl restart misskey
+
+# OpenRC
+rc-service misskey restart
+```