Add Gem & Bundler instructions to README
Add Gem & Bundler instructions to README

Previously there was no mention of actually installing the gem, which is usually useful for a lot of more junior devs.
This PR just adds a few general lines making the name of the gem `prometheus-client` explicit and giving Gemfile and global installation examples.

Signed-off-by: Christopher Guess <5142f4103c6858f6e53c9f1e6a287128934367fa@gmail.com>
prometheus/client_ruby