README.md in strong_migrations-0.6.2 vs README.md in strong_migrations-0.6.3

@@ -301,12 +301,22 @@
     change_column :users, :some_column, :new_type
   end
 end
 ```
 
-One exception is changing a `varchar` column to `text`, which is safe in Postgres.
+A few changes are safe in Postgres:
 
+- Changing between `varchar` and `text` columns
+- Increasing the precision of a `decimal` or `numeric` column
+- Making a `decimal` or `numeric` column unconstrained
+- Changing between `timestamp` and `timestamptz` columns when session time zone is UTC in Postgres 12+
+
+And a few in MySQL and MariaDB:
+
+- Increasing the length of a `varchar` column from under 255 up to 255
+- Increasing the length of a `varchar` column over 255
+
 #### Good
 
 A safer approach is to:
 
 1. Create a new column
@@ -688,7 +698,14 @@
 
 ```sh
 git clone https://github.com/ankane/strong_migrations.git
 cd strong_migrations
 bundle install
+
+# Postgres
+createdb strong_migrations_test
 bundle exec rake test
+
+# MySQL and MariaDB
+mysqladmin create strong_migrations_test
+ADAPTER=mysql2 bundle exec rake test
 ```