Document opinion: order of migration matters, hence linear

Imagine you have the table `create table my_table (id serial primary key);` and these two migrations on separate branches:

```sql
alter table my_table add column column_1 int;
```

```sql
alter table my_table add column column_a text;
```

At first you might think these don't conflict, but actually they do, the end result is dependent on order:

```sql
create table my_table (
id serial primary key,
column_1 int,
column_a text
);
```

versus:

```sql
create table my_table (
id serial primary key,
column_a text,
column_1 int
);
```

Now if you try and represent this table value as a tuple - `(1, 2, 'three')::my_table` - then this will work against one DB but fail against the other. Order of columns is significant.

So it's best to ensure that your migrations are completely linear.