Add rationale for commenting out s.extensions

Just a quick note for future us on _why_ this is commented out.

puma.gemspec

@@ -10,7 +10,11 @@ Gem::Specification.new do |s|
   s.summary = "Puma is a simple, fast, threaded, and highly concurrent HTTP 1.1 server for Ruby/Rack applications"
   s.email = ["evan@phx.io"]
   s.executables = ["puma", "pumactl"]
-  #s.extensions = ["ext/puma_http11/extconf.rb"]
+
+  ##
+  # disable the C native extension part of this Gem---we want JRuby to be the default
+  # so we get that version when using a :git Gemfile dependency
+  # s.extensions = ["ext/puma_http11/extconf.rb"]
   s.add_runtime_dependency "nio4r", "~> 2.0"
   s.metadata["msys2_mingw_dependencies"] = "openssl"
   s.files = `git ls-files -- bin docs ext lib tools`.split("\n") +