Add pre-commit.ci. Solves #222.

Author: samcunliffe

docs/pages/ci.md

@@ -11,6 +11,7 @@ layout: default
 | [AppVeyor](https://www.appveyor.com/docs/)                                            | Continuous integration and continuous delivery platform.                          | 🟠  |
 | [Bamboo](https://confluence.atlassian.com/bamboo/bamboo-documentation-289276551.html) | Atlassian continuous integration and continuous delivery platform.                | 🟠  |
 | [Travis CI](https://docs.travis-ci.com/)                                              | Continuous integration and continuous delivery platform.                          | 🟠  |
+| [pre-commmit.ci](https://pre-commit.ci/)                                              | A bot that adds a pre-commit job to your GitHub Actions CI, and can automatically fix most trivial linting failures. Free for open-source projects. | 🟢  |
 
 <details>
 <summary> 🟢 explanation</summary>