Update installation documentation

Include new `jekyll-remote-theme` method for using Ruby gem version of the theme with GitHub Pages hosted sites.

ref: mmistakes/minimal-mistakes#1339

minimal-mistakes/README.md

@@ -65,34 +65,60 @@ Additional sample posts are available under [posts archive][year-archive] on the
 [tags-archive]: https://mmistakes.github.io/minimal-mistakes/tags/
 [year-archive]: https://mmistakes.github.io/minimal-mistakes/year-archive/
 
-## Usage
-
-For detailed instructions on how to configure, customize, add content, and more read the [theme's documentation](https://mmistakes.github.io/minimal-mistakes/docs/quick-start-guide/).
+## Installation
 
-**Note:** Gem version of the theme requires Jekyll v3.3+ and is not currently supported on [GitHub Pages](https://pages.github.com/). You can still use the theme with GitHub Pages, you'll just need to install using the old "[repo fork method](https://mmistakes.github.io/minimal-mistakes/docs/installation/)".
+There are three ways to install the theme: as a Ruby gem (for self-hosted sites), as a Ruby gem + jekyll-remote-theme plugin (GitHub Pages hosted sites), or forking/directly copying all of the theme files into your project.
 
-## Quick Start
+### Ruby Gem Method
 
-Add this line to your Jekyll site's `Gemfile`:
+1. Install the theme as a Ruby Gem by adding it to your `Gemfile` like so:
 
-```ruby
-gem "minimal-mistakes-jekyll"
-```
+   ```ruby
+   gem "minimal-mistakes-jekyll"
+   ```
 
-Add this line to your Jekyll site's `_config.yml` file:
+2. Fetch and update bundled gems by running the following [Bundler](http://bundler.io/) command:
 
-```yaml
-theme: minimal-mistakes-jekyll
-```
+   ```bash
+   bundle
+   ```
 
-Then run Bundler to install the theme gem and dependencies:
+3. Set the `theme` in your project's Jekyll `_config.yml` file:
 
-```bash
-bundle install
-```
+   ```yaml
+   theme: minimal-mistakes-jekyll
+   ```
 
 To update the theme run `bundle update`.
 
+## GitHub Pages Compatible Methods
+
+1. Create/replace the contents of your `Gemfile` with the following:
+
+   ```ruby
+   source "https://rubygems.org"
+
+   gem "github-pages", group: :jekyll_plugins
+   gem "jekyll-remote-theme"
+   ```
+
+2. Fetch and update bundled gems by running the following [Bundler](http://bundler.io/) command:
+
+   ```bash
+   bundle
+   ```
+
+3. Add `jekyll-remote-theme` to the `plugins` (previously gems) array in your `_config.yml` file:
+
+   ```yaml
+   plugins:
+     - jekyll-remote-theme
+   ```
+
+## Usage
+
+For detailed instructions on how to configure, customize, add/migrate content, and more read the [theme's documentation](https://mmistakes.github.io/minimal-mistakes/docs/quick-start-guide/).
+
 ---
 
 ## Contributing