Update README and init.rb: specify licenses and bump Redmine version requirement to 4.2.0, update plugin version to 6.0.0.

Author: cat-in-136

README.rdoc

@@ -24,6 +24,7 @@ Bug reports and pull requests are welcome on GitHub at https://github.com/cat-in
 
 == License
 
-MIT License.
-See the 'LICENSE.md' file.
+* Codes: MIT License.
+* Legacy Icons: famfamfam-silk: {CC-By-2.5}[https://creativecommons.org/licenses/by/2.5/] by Mark James http://www.famfamfam.com/lab/icons/silk/
+* Vector Icons: {Tabler Icons}[https://tabler.io/]: MIT License
 

init.rb

@@ -2,12 +2,12 @@
 require File.expand_path('../lib/redmine_scheduling_poll/hooks', __FILE__)
 
 Redmine::Plugin.register :redmine_scheduling_poll do
-  requires_redmine :version_or_higher => '4.0.0'
+  requires_redmine :version_or_higher => '4.2.0'
 
   name 'Scheduling Poll plugin'
   author '@cat_in_136'
   description 'provide simple polls to scheduling appointments'
-  version '5.0.0'
+  version '6.0.0'
   url 'https://github.com/cat-in-136/redmine_scheduling_poll'
   author_url 'https://github.com/cat-in-136/'