Updated Ruby README with more details on getting started.

Change-Id: I54df314660cdb861ad8c4da75a08d4cb97faf638

ruby/README.md

@@ -7,8 +7,51 @@ 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
-------------
+Installation from Gem
+---------------------
+
+When we release a version of Protocol Buffers, we will upload a Gem to
+[RubyGems](https://www.rubygems.org/). To use this pre-packaged gem, simply
+install it as you would any other gem:
+
+    $ gem install [--prerelease] google-protobuf
+
+The `--pre` flag is necessary if we have not yet made a non-alpha/beta release
+of the Ruby extension; it allows `gem` to consider these "pre-release"
+alpha/beta versions.
+
+Once the gem is installed, you may or may not need `protoc`. If you write your
+message type descriptions directly in the Ruby DSL, you do not need it.
+However, if you wish to generate the Ruby DSL from a `.proto` file, you will
+also want to install Protocol Buffers itself, as described in this repository's
+main `README` file. The version of `protoc` included in the latest release
+supports the `--ruby_out` option to generate Ruby code.
+
+A simple example of using the Ruby extension follows. More extensive
+documentation may be found in the RubyDoc comments (`call-seq` tags) in the
+source, and we plan to release separate, more detailed, documentation at a
+later date.
+
+    require 'google/protobuf'
+
+    # generated from my_proto_types.proto with protoc:
+    #  $ protoc --ruby_out=. my_proto_types.proto
+    require 'my_proto_types'
+
+    mymessage = MyTestMessage.new(:field1 => 42, :field2 => ["a", "b", "c"])
+    mymessage.field1 = 43
+    mymessage.field2.push("d")
+    mymessage.field3 = SubMessage.new(:foo => 100)
+
+    encoded_data = MyTestMessage.encode(mymessage)
+    decoded = MyTestMessage.decode(encoded_data)
+    assert decoded == mymessage
+
+    puts "JSON:"
+    puts MyTestMessage.encode_json(mymessage)
+
+Installation from Source (Building Gem)
+---------------------------------------
 
 To build this Ruby extension, you will need: