Update notes.

README.md

+# Some notes
+
+  * http://gunicorn.org/#quickstart
+  * http://guides.rubygems.org/gems-with-extensions/
+  * https://bundler.io/v1.12/guides/creating_gem.html

ext/Gemfile

-# frozen_string_literal: true
-source "https://rubygems.org"
-
-gem 'prometheus-client', git: 'https://gitlab.ie.suberic.net/p4g/client_ruby.git', :branch => 'ext'

ext/run.sh

-#! /bin/sh
-
-test -f Gemfile.lock && rm Gemfile.lock
-DEBUG_RESOLVER=1 bundle install

puma/Gemfile

-# frozen_string_literal: true
-source "https://rubygems.org"
-
-gem 'puma', git: 'https://github.com/puma/puma.git', :branch => 'master'

puma/run.sh

-#! /bin/sh
-
-test -f Gemfile.lock && rm Gemfile.lock
-DEBUG_RESOLVER=1 bundle install