Provide a Ruby extension.

This adds a Ruby extension in ruby/ that is based on the 'upb' library
(now included as a submodule), and adds support for Ruby code generation
to the protoc compiler.

ruby/README.md

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+This directory contains the Ruby extension that implements Protocol Buffers
+functionality in Ruby.
+
+The Ruby extension makes use of generated Ruby code that defines message and
+enum types in a Ruby DSL. You may write definitions in this DSL directly, but
+we recommend using protoc's Ruby generation support with .proto files. The
+build process in this directory only installs the extension; you need to
+install protoc as well to have Ruby code generation functionality.
+
+Installation
+------------
+
+To build this Ruby extension, you will need:
+
+* Rake
+* Bundler
+* Ruby development headers
+* 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:
+
+    $ sudo gem install bundler rake rake-compiler rspec rubygems-tasks
+
+Then build the Gem:
+
+    $ rake gem
+    $ gem install pkg/protobuf-$VERSION.gem