Clarify README instructions for ActiveRecord

* If you're using the ActiveRecord adapter, your transition class
should `include Statesman::Adapters::ActiveRecordTransition`.
* If you're using a PostgreSQL JSON column to store metadata, you
won't be able to
`include Statesman::Adapters::ActiveRecordTransition` as there is
an incompatability (which we'll aim to resolve soon, see #309).
Without this module, if you want to customise your transition
class's "updated timestamp column", you will need to define
`.updated_timestamp_column` yourself.

README.md

@@ -121,7 +121,7 @@ Order.not_in_state(:checking_out) # => [#<Order id: "123">]
 
 By default Statesman stores transition history in memory only. It can be
 persisted by configuring Statesman to use a different adapter. For example,
-ActiveRecord within Rails:
+for ActiveRecord within Rails:
 
 `config/initializers/statesman.rb`:
 
@@ -137,6 +137,10 @@ Generate the transition model:
 $ rails g statesman:active_record_transition Order OrderTransition
 ```
 
+Your transition class should
+`include Statesman::Adapters::ActiveRecordTransition` if you're using the
+ActiveRecord adapter.
+
 If you're using the ActiveRecord adapter and decide not to include the default
 `updated_at` column in your transition table, you'll need to configure the
 `updated_timestamp_column` option on the transition class, setting it to another column
@@ -178,8 +182,12 @@ or 5. To do that
   t.json :metadata, default: {}
   ```
 
-* Remove `include Statesman::Adapters::ActiveRecordTransition` statement from your
-  transition model
+* Remove the `include Statesman::Adapters::ActiveRecordTransition` statement from
+  your transition model. (If you want to customise your transition class's "updated
+  timestamp column", as described above, you should define a
+  `.updated_timestamp_column` method on your class and return the name of the column
+  as a symbol, or `nil` if you don't want to record an updated timestamp on
+  transitions.)
 
 ## Configuration