Add upgrade from legacy doc in README (#204)

Co-authored-by: Astrid <voltasalt@gmail.com>

README.md

@@ -54,6 +54,32 @@ $ docker-compose up -d
 
 (tip: use `scripts/run-test-db.sh` to run a temporary PostgreSQL database on your local system. Requires Docker.)
 
+# Upgrading database from legacy version
+There are a few adjustments to do for the new versions of PluralKit to work with a legacy PluralKit database :
+
+* Dump the database (you never know what may happen !)
+* Upgrade the database by firing those lines in psql (or by putting them in an sql file and giving it to postgres) :
+```sql
+do $$ begin
+    create type proxy_tag as (
+        prefix text,
+        suffix text
+    );
+exception when duplicate_object then null;
+end $$;
+
+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;
+update members set proxy_tags = array[(members.prefix, members.suffix)]::proxy_tag[] where members.prefix is not null or members.suffix is not null;
+alter table members drop column IF EXISTS prefix cascade;
+alter table members drop column IF EXISTS suffix cascade;
+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[];
+```
+* Start PluralKit and let it finish the automatic database upgrades 
+
 # Documentation
 See [the docs/ directory](./docs/README.md)