Add spec & README for completions

Author: alexrudall

README.md

@@ -399,6 +399,20 @@ end
 # => "The weather is nice 🌞"
 ```
 
+### Completions
+
+Hit the OpenAI API for a completion using other GPT-3 models:
+
+````ruby
+response = client.completions(
+    parameters: {
+        model: "text-davinci-001",
+        prompt: "Once upon a time",
+        max_tokens: 5
+    })
+puts response["choices"].map { |c| c["text"] }
+# => [", there lived a great"]
+
 ### Edits
 
 Send a string and some instructions for what to do to the string:
@@ -413,7 +427,7 @@ response = client.edits(
 )
 puts response.dig("choices", 0, "text")
 # => What day of the week is it?
-```
+````
 
 ### Embeddings
 

spec/fixtures/cassettes/gpt-3_5-turbo-instruct_completions_once_upon_a_time.yml

@@ -0,0 +1,27 @@
+RSpec.describe OpenAI::Client do
+  describe "#completions: gpt-3.5-turbo-instruct" do
+    context "with a prompt and max_tokens", :vcr do
+      let(:prompt) { "Once upon a time" }
+      let(:max_tokens) { 5 }
+
+      let(:response) do
+        OpenAI::Client.new.completions(
+          parameters: {
+            model: model,
+            prompt: prompt,
+            max_tokens: max_tokens
+          }
+        )
+      end
+      let(:text) { response.dig("choices", 0, "text") }
+      let(:cassette) { "#{model} completions #{prompt}".downcase }
+      let(:model) { "gpt-3.5-turbo-instruct" }
+
+      it "succeeds" do
+        VCR.use_cassette(cassette) do
+          expect(text.split.empty?).to eq(false)
+        end
+      end
+    end
+  end
+end