README: Add withTransaction to the docs.

README.md

@@ -18,7 +18,7 @@ module Test.Example where
 
 import Prelude
 
-import Database.PostgreSQL (defaultPoolConfiguration, command, execute, newPool, query, Query(Query), withConnection)
+import Database.PostgreSQL (defaultPoolConfiguration, command, execute, newPool, query, Query(Query), withConnection, withTransaction)
 import Database.PostgreSQL.Row (Row0(Row0), Row3(Row3))
 import Data.Decimal as Decimal
 import Data.Maybe (Maybe(..))
@@ -63,7 +63,16 @@ The last `Row0` value indicates that this `Query` doesn't take any additional pa
     """) Row0
 ```
 
-We can insert some data calling `execute` function with `INSERT` statement.
+There is `withTransaction` helper provided. You can wrap the whole
+interaction with database in it. It will rollback if any exception
+is thrown during execution of given `Aff` block. It commits in the other case.
+
+
+```purescript
+    withTransaction conn $ do
+```
+
+Now we can insert some data calling `execute` function with `INSERT` statement.
 Please notice that we are passing a tuple of arguments to this query
 using dedicated constructor. In this case `Row3`.
 This library provides types from `Row0` to `Row19` and they are wrappers which
@@ -87,7 +96,6 @@ verbose but is not restricted to limited and constant number of arguments.
         INSERT INTO fruits (name, delicious, price)
         VALUES ($1, $2, $3)
       """) ("lemon" /\ false /\ Decimal.fromString "3.30")
-
 ```
 
 Of course `Row*` types and nested tuples can be also used when we are fetching
@@ -106,14 +114,13 @@ data from db.
 
 There is also a `command` function at our disposal.
 Some postgres SQL expressions return a "command tag" which carries
-a value of rows which were affected by a given query.
+a value with a number of rows which were affected by a given query.
 For example we can have: `DELETE rows`, `UPDATE rows`, `INSERT oid rows` etc.
 This function should return `rows` value associated with given response.
 
 ```purescript
       deleted <- command conn (Query """DELETE FROM fruits """) Row0
       liftEffect <<< assert $ deleted == 2
-
 ```
 
 ## Testing