README: Explain the use of Gemfiles for older rubies

README.md

@@ -36,7 +36,9 @@ Because software tools are for people. If you don’t care, why are you contribu
 
 ## Installation
 
-	[sudo] gem install github_changelog_generator
+	gem install github_changelog_generator
+
+See also Troubleshooting.
 
 ## Output example
 
@@ -223,6 +225,31 @@ If you're seeing this warning, please do the following:
 1. Make sure you're providing an OAuth token, so you're not making requests anonymously. Using an OAuth token increases your hourly request maximum from 60 to 5000.
 2. If you have a large repo with lots of issues/PRs, you can use `--max-issues NUM` to limit the number of issues that are pulled back. For example: `--max-issues 1000`
 
+- ***My Ruby version is very old, can I use this?***
+
+When your Ruby is old, and you don't want to upgrade, and your want to
+control which libraries you use, you can use Bundler.
+
+In a Gemfile, perhaps in a non-deployed `:development` group, add this
+gem:
+
+```ruby
+group :development do
+  gem 'github_changelog_generator', require: false
+end
+```
+
+Then you can keep back dependencies like rack, which currently is only
+compatible with Ruby >= 2.2.2. So, use an older version for your app by
+adding a line like this to the Gemfile:
+
+```
+gem 'rack', '~> 1.6'
+```
+
+This way, you can keep on using github_changelog_generator, even if you
+can't get the latest version of Ruby installed.
+
 ## Contributing
 
 1. Create an issue and describe your idea