docs: Add section on HTTPS support for the deno serve CLI subcommand

Author: adamgreg

runtime/reference/cli/serve.md

@@ -139,6 +139,15 @@ specific interface:
 deno serve --host=127.0.0.1 server.ts
 ```
 
+## HTTPS support
+
+HTTPS can be enabled by providing the server TLS certificate and private key
+pair as PEM-encoded files:
+
+```sh
+deno serve --tls-cert cert.pem --tls-key key.pem
+```
+
 ## Horizontal scaling
 
 Run multiple server instances across CPU cores for better throughput: