add Tidelift badge and reference to README

[skip ci]

README.md

@@ -19,6 +19,7 @@ or CSS3 selectors.
 [![Concourse CI](https://ci.nokogiri.org/api/v1/teams/nokogiri-core/pipelines/nokogiri/jobs/ruby-2.4-system/badge)](https://ci.nokogiri.org/teams/nokogiri-core/pipelines/nokogiri?groups=master)
 [![Code Climate](https://codeclimate.com/github/sparklemotion/nokogiri.svg)](https://codeclimate.com/github/sparklemotion/nokogiri)
 [![Join the chat at https://gitter.im/sparklemotion/nokogiri](https://badges.gitter.im/sparklemotion/nokogiri.svg)](https://gitter.im/sparklemotion/nokogiri?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Tidelift dependencies](https://tidelift.com/badges/github/sparklemotion/nokogiri)](https://tidelift.com/subscription/pkg/rubygems-nokogiri?utm_source=rubygems-nokogiri&utm_medium=referral&utm_campaign=readme)
 
 
 ## Features
@@ -75,6 +76,10 @@ There are open-source tutorials (to which we invite contributions!) here: http:/
 * Before filing a bug report, please read our submission guidelines: http://nokogiri.org/tutorials/getting_help.html
 * The IRC channel is #nokogiri on freenode.
 
+Consider subscribing to [Tidelift][tidelift] which provides license assurances and timely security notifications for your open source dependencies, including Nokogiri. [Tidelift][tidelift] subscriptions also help the Nokogiri maintainers fund our [automated testing](https://ci.nokogiri.org) which in turn allows us to ship releases, bugfixes, and security updates more often.
+
+  [tidelift]: https://tidelift.com/subscription/pkg/rubygems-nokogiri?utm_source=rubygems-nokogiri&utm_medium=referral&utm_campaign=readme
+
 
 ## Security and Vulnerability Reporting