Move legacy migration docs to a separate file and reword

LEGACYMIGRATE.md

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+# Legacy bot migration
+Until the introduction of the database migration system around December 2019, migrations were done manually.
+
+To bridge the gap between the `legacy` branch's database schema and something the modern migration system can work with, run the following SQL commands on the database:
+
+```sql
+-- Create the proxy_tag type
+do $$ begin
+    create type proxy_tag as (
+        prefix text,
+        suffix text
+    );
+exception when duplicate_object then null;
+end $$;
+
+-- Add new columns to `members`
+alter table members add column IF NOT EXISTS display_name text;
+alter table members add column IF NOT EXISTS proxy_tags proxy_tag[] not null default array[]::proxy_tag[];
+alter table members add column IF NOT EXISTS keep_proxy bool not null default false;
+
+-- Transfer member proxy tags from the `prefix` and `suffix` columns to the `proxy_tags` array
+update members set proxy_tags = array[(members.prefix, members.suffix)]::proxy_tag[]
+    where members.prefix is not null or members.suffix is not null;
+
+-- Add other columns
+alter table messages add column IF NOT EXISTS original_mid bigint;
+alter table servers add column IF NOT EXISTS log_blacklist bigint[] not null default array[]::bigint[];
+alter table servers add column IF NOT EXISTS blacklist bigint[] not null default array[]::bigint[];
+
+-- Drop old proxy tag columns
+alter table members drop column IF EXISTS prefix cascade;
+alter table members drop column IF EXISTS suffix cascade;
+```
+
+You should probably take a database backup before doing any of this.
+
+The .NET version of the bot should pick up on any further migrations from this point :)