Added building/packaging instructions to the README.

Author: jdantonio

README.md

@@ -35,7 +35,14 @@
   </tr>
 </table>
 
-## Install
+## Installing and Building
+
+This gem includes several platform-specific optimizations. To reduce the possibility of
+compilation errors, we provide pre-compiled gem packages for several platforms as well
+as a pure-Ruby build. Installing the gem should be no different than installing any other
+Rubygems-hosted gem.
+
+### Installing
 
 ```shell
 gem install concurrent-ruby
@@ -49,7 +56,32 @@ gem 'concurrent-ruby'
 
 and run `bundle install` from your shell.
 
-_NOTE: There is an old gem from 2007 called "concurrent" that does not appear to be under active development. That isn't us. Please do not run* `gem install concurrent`*. It is not the droid you are looking for._
+### Building
+
+Because we provide pre-compiled gem builds, users should never need to build the gem manually.
+The build process for this gem is completely automated using open source tools. All of
+the automation components are available in the [ruby-concurrency/rake-compiler-dev-box](https://github.com/ruby-concurrency/rake-compiler-dev-box)
+GitHub repository.
+
+This gem will compile native C code under MRI and native Java code under JRuby. It is
+also possible to build a pure-Ruby version. All builds have identical functionality.
+The only difference is performance. Additionally, pure-Ruby classes are always available,
+even when using the native optimizations. Please see the [documentation](http://ruby-concurrency.github.io/concurrent-ruby/)
+for more details.
+
+To build and package the gem using MRI or JRuby, install the necessary build dependencies and run:
+
+```shell
+bundle exec rake compile
+bundle exec rake build
+```
+
+To build and package a pure-Ruby gem, on *any* platform and interpreter
+(including MRI and JRuby), run:
+
+```shell
+BUILD_PURE_RUBY='true' bundle exec rake build
+```
 
 ## Features & Documentation