update bootstrap v4.1 generator template (#1569)

simple form now 💜 bootstrap 4.1

The bootstrap generator is extracted from:
heartcombo/simple_form-bootstrap#56

* supported form wrapper all covered by *tests*
  * vertical form
  * horizontal form
  * inline form
  * custom fields form
  * input groups form
  * floating labels form

:link: heartcombo/simple_form-bootstrap#56
🔗 #1561

Resolves #1337

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## Unreleased
 
+* Add bootstrap v4.1 generator template. [@m5o](https://github.com/m5o)
 * Add Rails 5.2 support. [@gobijan](https://github.com/gobijan)
 * Add API to register custom components.[@feliperenan](https://github.com/feliperenan)
 * Allow custom errors classes to inputs.[@feliperenan](https://github.com/feliperenan)
@@ -23,7 +24,7 @@
 * Add frozen_string_literal magic comment to several files. [@oniofchaos](https://github.com/oniofchaos)
 * Try convert @object to model in case we got decorated object [@timurvafin](https://github.com/timurvafin)
 - From now, if you are using some object that inherits from `SimpleDelegator`, you must implement
-  `def to_model; self; end`. Otherwise, *Simple Form* will convert the decorated object to the model 
+  `def to_model; self; end`. Otherwise, *Simple Form* will convert the decorated object to the model
   since `SimpleDelegator` will delegate it to the model.
 * Code cleanup [@Fornacula](https://github.com/Fornacula)
 

lib/generators/simple_form/templates/README

@@ -3,9 +3,9 @@
   Be sure to have a copy of the Bootstrap stylesheet available on your
   application, you can get it on http://getbootstrap.com/.
 
-  Inside your views, use the 'simple_form_for' with one of the Bootstrap form
-  classes, '.form-horizontal' or '.form-inline', as the following:
+  Inside your views, use the 'simple_form_for' with the Bootstrap form
+  class, '.form-inline', as the following:
 
-    = simple_form_for(@user, html: { class: 'form-horizontal' }) do |form|
+    = simple_form_for(@user, html: { class: 'form-inline' }) do |form|
 
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