README.md in config-1.5.1 vs README.md in config-1.6.0

@@ -297,9 +297,52 @@
 raise a `Config::Validation::Error` containing a nice message with information about all the mismatches
 between the schema and your config.
 
 Check [dry-validation](https://github.com/dry-rb/dry-validation) for more details.
 
+### Missing keys
+
+For an example settings file:
+
+```yaml
+size: 1
+server: google.com
+```
+
+You can test if a value was set for a given key using `key?` and its alias `has_key?`:
+
+```ruby
+Settings.key?(:path)
+# => false
+Settings.key?(:server)
+# => true
+```
+
+By default, accessing to a missing key returns `nil`:
+
+```ruby
+Settings.key?(:path)
+# => false
+Settings.path
+# => nil
+```
+
+This is not "typo-safe". To solve this problem, you can configure the `fail_on_missing` option:
+
+```ruby
+Config.setup do |config|
+  config.fail_on_missing = true
+  # ...
+end
+```
+
+So it will raise a `KeyError` when accessing a non-existing key (similar to `Hash#fetch` behaviour):
+
+```ruby
+Settings.path
+# => raises KeyError: key not found: :path
+```
+
 ### Environment variables
 
 See section below for more details.
 
 ## Working with environment variables