feat(guides): add auth guide (#5)

* fix(readme): update it and mark as beta

* add cloud link

* readme update

* feat(guides): add auth guide

Author: Pothulapati

docs/guides/auth.md

@@ -0,0 +1,15 @@
+# Authentication Guide
+
+Visit [dragonflydb.cloud](https://dragonflydb.cloud) and generate an API key with the required permissions. Then set the following environment variable before using the Terraform provider:
+
+```bash
+export DFCLOUD_API_KEY=<YOUR_API_KEY>
+```
+
+The same API key can also be set in the provider configuration block in the Terraform configuration file:
+
+```hcl
+provider "dfcloud" {
+  api_key = "<YOUR_API_KEY>"
+}
+```