Add post_install_message in gemspec

ddtrace.gemspec

@@ -75,4 +75,15 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'datadog-ci', '~> 0.8.1'
 
   spec.extensions = ['ext/datadog_profiling_native_extension/extconf.rb', 'ext/datadog_profiling_loader/extconf.rb']
+
+  spec.post_install_message = <<-MSG
+    Thank you for installing ddtrace.
+
+    As of version 2, the `ddtrace` gem has been renamed to `datadog` to better represent the full suite of Datadog products.
+    The 1.x series will now only receive maintenance updates for security and critical bug fixes.
+
+    To upgrade, please update your Gemfile to replace `ddtrace` with `datadog`.
+
+    For detailed instructions on migration, see: https://github.com/DataDog/dd-trace-rb/blob/master/docs/UpgradeGuide2.md
+  MSG
 end

spec/ddtrace/release_gem_spec.rb

@@ -131,5 +131,11 @@
         end
       end
     end
+
+    describe '#post_install_message' do
+      it do
+        expect(gemspec.post_install_message).to_not be_nil
+      end
+    end
   end
 end