Add README.md (#2)

Author: quentindemetz

README.md

@@ -0,0 +1,57 @@
+# Batch Update
+
+Update multiple records with different values in an optimized number of queries.
+
+This differs from [activerecord-import](https://github.com/zdennis/activerecord-import) because the latter issues a `INSERT ... ON DUPLICATE KEY UPDATE` statement which re-inserts the record if it happens to have been deleted in a other thread.
+
+## Usage
+Include in your Gemfile: `gem 'batch_update'`
+
+```ruby
+cat1 = Cat.create!(name: 'Felix', birthday: '1990-03-13')
+cat2 = Cat.create!(name: 'Garfield', birthday: '1978-06-19')
+cat3 = Cat.create!(name: 'Daisy', birthday: '1970-01-01')
+
+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
+```
+
+The SQL query looks like the following:
+```SQL
+WITH "batch_updates" (birthday, id) AS (
+  VALUES (CAST('2024-01-01' AS date), CAST(1 AS INTEGER)), ('2024-06-06', 2), ('1900-01-01', 3),
+)
+UPDATE "cats"
+SET
+  "birthday" = "batch_updates"."birthday"
+FROM "batch_updates"
+WHERE
+  "cats"."id" = "batch_updates"."id"
+```
+
+## Advanced usage
+Specify which columns to update (all columns are included by default):
+```ruby
+cat1.name = 'Lilly'
+cat1.birthday = '2024-01-01'
+cat2.birthday = '2023-06-06'
+Cat.batch_update([cat1, cat2], columns: %i[birthday])
+```
+
+Ignore model validations (all validations run by default):
+```ruby
+cat1.name = ''
+cat2.name = ''
+Cat.batch_update([cat1, cat2], validate: false)
+```
+
+Specify a different batch size (100 by default):
+```
+Cat.batch_update(cats, batch_size: 1000)
+```
+
+## License
+MIT