Link to sentry website (#21)

Fixes #20

Author: mwarkentin

README.md

@@ -2,7 +2,7 @@
 [![CircleCI](https://circleci.com/gh/jianyuan/terraform-provider-sentry/tree/master.svg?style=svg)](https://circleci.com/gh/jianyuan/terraform-provider-sentry/tree/master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/jianyuan/terraform-provider-sentry)](https://goreportcard.com/report/github.com/jianyuan/terraform-provider-sentry)
 
-Terraform provider for Sentry
+Terraform provider for [Sentry](https://sentry.io)
 
 ## Usage