Add Params Is A Hash With Indifferent Access as a Rails TIL

rails/params-is-a-hash-with-indifferent-access.md

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+# Params Is A Hash With Indifferent Access
+
+When you have an instance of `ActionController::Parameters`—which you often do
+with `params` in a Rails controller—under the hood you have a
+`HashWithIndifferentAccess`.
+
+Here is what the initializer looks like when you call
+`ActionController::Parameters.new(some_hash)`.
+
+```ruby
+def initialize(parameters = {}, logging_context = {})
+  @parameters = parameters.with_indifferent_access
+  @logging_context = logging_context
+  @permitted = self.class.permit_all_parameters
+end
+```
+
+This means you can reference the keys in your parameters as either a _string_
+key or a _symbol_ key.
+
+```ruby
+> params = ActionController::Parameters.new({ username: 'tacocat' })
+=> #<ActionController::Parameters {"username"=>"tacocat"} permitted: false>
+> params['username']
+=> "tacocat"
+> params[:username]
+=> "tacocat"
+```
+
+Note that `ActiveSupport::HashWithIndifferentAccess` is not an ancestor of the
+`ActionController::Parameters` class (like it once used to be), but is just how
+the incoming hash is transformed when initialized.