Add USAGE.md

Author: ProZsolt

USAGE.md

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+# Usage
+
+Usage examples for SendGrid ruby-http-client
+
+## Initialization
+
+```
+require_relative '../lib/ruby_http_client'
+
+# This uses the SendGrid API as an example
+headers = JSON.parse('
+  {
+    "Authorization": "Bearer ' + ENV['SENDGRID_API_KEY'] + '"
+  }
+')
+host = 'https://api.sendgrid.com'
+client = SendGrid::Client.new(host: host, request_headers: headers)
+```
+
+## Table of Contents
+
+- [GET](#get)
+- [DELETE](#delete)
+- [POST](#post)
+- [PUT](#put)
+- [PATCH](#patch)
+
+<a name="get"></a>
+## GET
+
+#### GET Collection
+
+```
+query_params = { 'limit' => 100, 'offset' => 0 }
+response = client.version('v3').api_keys.get(query_params: query_params)
+puts response.status_code
+puts response.body
+puts response.headers
+```
+
+#### GET Single
+
+```
+response = client.version('v3').api_keys._(api_key_id).get
+puts response.status_code
+puts response.body
+puts response.headers
+```
+
+<a name="delete"></a>
+## DELETE
+
+```
+response = client.api_keys._(api_key_id).delete
+puts response.status_code
+puts response.headers
+```
+
+<a name="post"></a>
+## POST
+
+```
+request_body = JSON.parse('
+    {
+        "name": "My API Key Ruby Test",
+        "scopes": [
+            "mail.send",
+            "alerts.create",
+            "alerts.read"
+        ]
+    }
+')
+response = client.version('v3').api_keys.post(request_body: request_body)
+puts response.status_code
+puts response.body
+puts response.headers
+api_key_id = JSON.parse(response.body)['api_key_id']
+```
+
+<a name="put"></a>
+## PUT
+
+```
+request_body = JSON.parse('
+    {
+        "name": "A New Hope",
+        "scopes": [
+            "user.profile.read",
+            "user.profile.update"
+        ]
+    }
+')
+response = client.api_keys._(api_key_id).put(request_body: request_body)
+puts response.status_code
+puts response.body
+puts response.headers
+```
+
+<a name="patch"></a>
+## PATCH
+
+```
+request_body = JSON.parse('
+    {
+        "name": "A New Hope"
+    }
+')
+response = client.api_keys._(api_key_id).patch(request_body: request_body)
+puts response.status_code
+puts response.body
+puts response.headers
+```