README.md update for Ruby gem.

ruby/README.md

@@ -18,13 +18,7 @@ To build this Ruby extension, you will need:
 * a C compiler
 * the upb submodule
 
-First, ensure that upb/ is checked out:
-
-    $ cd ..   # top level protobuf directory
-    $ git submodule init
-    $ git submodule update
-
-Then install the required Ruby gems:
+First, install the required Ruby gems:
 
     $ sudo gem install bundler rake rake-compiler rspec rubygems-tasks
 
@@ -32,3 +26,7 @@ Then build the Gem:
 
     $ rake gem
     $ gem install pkg/protobuf-$VERSION.gem
+
+This gem includes the upb parsing and serialization library as a single-file
+amalgamation. It is up-to-date with upb git commit
+`535bc2fe2f2b467f59347ffc9449e11e47791257`.