README: columns argument is mandatory (#8)

Author: quentindemetz

README.md

@@ -16,7 +16,7 @@ cat1.birthday = '2024-01-01'
 cat2.birthday = '2024-06-06'
 cat3.birthday = '1900-01-01'
 
-Cat.batch_update([cat1, cat2, cat3]) # issues a single SQL query
+Cat.batch_update([cat1, cat2, cat3], columns: :all) # issues a single SQL query
 ```
 
 The SQL query looks like the following:
@@ -45,12 +45,12 @@ Ignore model validations (all validations run by default):
 ```ruby
 cat1.name = ''
 cat2.name = ''
-Cat.batch_update([cat1, cat2], validate: false)
+Cat.batch_update([cat1, cat2], columns: :all, validate: false)
 ```
 
 Specify a different batch size (100 by default):
-```
-Cat.batch_update(cats, batch_size: 1000)
+```ruby
+Cat.batch_update(cats, columns: :all, batch_size: 1000)
 ```
 
 ## License