Convert README from rdoc to markdown

Also add CI badges

pg-ldap-sync.gemspec

@@ -18,6 +18,7 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+  spec.rdoc_options = %w[--main README.md --charset=UTF-8]
 
   spec.add_runtime_dependency "net-ldap", "~> 0.16"
   spec.add_runtime_dependency "kwalify", "~> 0.7"