Merge pull request #5 from Shopify/pz-readme

Write README.md

Author: peterzhu2118

README.md

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+# shopify-ruby-definitions
+
+This repository contains [ruby-build](https://github.com/rbenv/ruby-build/) definitions of Rubies we use at Shopify. These builds are generally API complient with release Rubies, but with additional backports for bugfixes and performance.
+
+## Quick start
+
+1. Install ruby-build by [following the instructions](https://github.com/rbenv/ruby-build/#installation).
+1. Install the gem:
+    ```
+    $ gem install shopify-ruby-definitions
+    ```
+1. Acquire the specific Ruby version of the Ruby version you want to install. Ruby 3.2 is used as an example here:
+    ```
+    $ export RUBY_VERSION=`ruby -rshopify_ruby_definitions -e 'puts ShopifyRubyDefinitions.resolve_version("3.2")'`
+    ```
+1. Set the definitions path for ruby-build:
+    ```
+    $ export RUBY_BUILD_DEFINITIONS=`ruby -rshopify_ruby_definitions -e 'puts ShopifyRubyDefinitions::RubyVersions::VERSIONS_DIRECTORY'`
+    ```
+1. Install the Ruby version using ruby-build:
+    ```
+    ruby-build $RUBY_VERSION
+    ```