Add Guard with bundler, rspec and rubocop plugins

Guardfile

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+guard :bundler do
+  watch('nostr.gemspec')
+end
+
+guard :bundler_audit, run_on_start: true do
+  watch('Gemfile.lock')
+end
+
+group :tests do
+  guard :rspec, all_on_start: true, cmd: 'COVERAGE=false bundle exec rspec --format progress' do
+    watch(%r{^spec/.+_spec\.rb$})
+    watch(%r{^lib/(.+)\.rb$}) { |m| "spec/#{m[1]}_spec.rb" }
+    watch('spec/spec_helper.rb') { 'spec' }
+  end
+end
+
+guard :rubocop do
+  watch(/.+\.rb$/)
+  watch(%r{(?:.+/)?\.rubocop\.yml$}) { |m| File.dirname(m[0]) }
+end