Docs: Encourage the use of the plugins config key

[first-timers]

🆕🐥☝ First Timers Only.

This issue is reserved for people who never contributed to Open Source before. We know that the process of creating a pull request is the biggest barrier for new contributors. This issue is for you 💝

About First Timers Only.

🎃💻👕 Hacktoberfest: Trick or Treat!

If you haven’t yet, sign up for Hacktoberfest to earn an exclusive T-Shirt. Plus I’m sure we can teach you a cool trick or two in the process

🤔 What you will need to know.

Nothing. This issue is meant to welcome you to Open Source :) We are happy to walk you through the process.

📋 Step by Step

• 🙋 Claim this issue: Comment below.

  Once claimed we add you as contributor to this repository.

• 👌 Accept our invitation to this repository. Once accepted, assign yourself to this issue

• 📝 Update the file README.md in the jekyll-redirect-from repository (press the little pen Icon) and edit the line as shown below.

@@ -34,9 +34,10 @@ Or install it yourself as:
 Once it's installed into your evironment, add it to your `_config.yml`:
 
 ```yaml
-gems:
+plugins:
   - jekyll-redirect-from

+:bulb: If you are using a Jekyll version less than 3.5.0, use the gems key instead of plugins.

If you're using Jekyll in safe mode to mimic GitHub Pages, make sure to
add jekyll-redirect-from to your whitelist:

- [ ] 💾 **Commit** your changes

- [ ] 🔀 **Start a Pull Request**. There are two ways how you can start a pull request:

1. If you are familiar with the terminal or would like to learn it, [here is a great tutorial](https://egghead.io/series/how-to-contribute-to-an-open-source-project-on-github) on how to send a pull request using the terminal.

2. You can [edit files directly in your browser](https://help.github.com/articles/editing-files-in-your-repository/)

- [ ] 🏁 **Done** Ask in comments for a review :)

### 🤔❓ Questions

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This issue was created by [First-Timers-Bot](https://github.com/hoodiehq/first-timers-bot).