Add Installation Instructions for Ubuntu (#6925)

Merge pull request 6925

docs/_docs/installation.md

@@ -4,12 +4,14 @@ description: Official guide to install Jekyll on macOS, GNU/Linux or Windows.
 permalink: /docs/installation/
 ---
 
-- [Requirements](#requirements)
-- [Install on macOS](#macOS)
-- [Install on Windows](../windows/)
-- [Upgrade Jekyll](#upgrade-jekyll)
+Jekyll is a [Ruby Gem](http://guides.rubygems.org/rubygems-basics/), and can be
+installed on most systems.
 
-Installing Jekyll should be straight-forward if your system meets the requirements.
+- [Requirements](#requirements)
+- [Install Jekyll on macOS](#macOS)
+- [Install Jekyll on Ubuntu Linux](#ubuntu)
+- [Install Jekyll on Windows](../windows/)
+- [Upgrade Jekyll](#upgrade-jekyll)
 
 ## Requirements
 
@@ -111,6 +113,35 @@ Check out the [troubleshooting](../troubleshooting/) page or
 
 </div>
 
+## Install on Ubuntu Linux {#ubuntu}
+
+Before we install Jekyll, we need to make sure we have all the required
+dependencies.
+
+```sh
+sudo apt-get install ruby ruby-dev build-essential
+```
+
+It is best to avoid installing Ruby Gems as the root user. Therefore, we need to
+set up a gem installation directory for your user account. The following
+commands will add environment variables to your `~/.bashrc` file to configure
+the gem installation path. Run them now:
+
+```sh
+echo '# Install Ruby Gems to ~/gems' >> ~/.bashrc
+echo 'export GEM_HOME=$HOME/gems' >> ~/.bashrc
+echo 'export PATH=$HOME/gems/bin:$PATH' >> ~/.bashrc
+source ~/.bashrc
+```
+
+Finally, install Jekyll:
+
+```sh
+gem install jekyll bundler
+```
+
+That's it! You're ready to start using Jekyll.
+
 ## Upgrade Jekyll
 
 Before you start developing with Jekyll, you may want to check that you're up to date with the latest version. To find the currently installed version of Jekyll, run one of these commands: