[docs] add http2 example (#1000)

examples/http2/README.md

@@ -0,0 +1,18 @@
+## How to generate RSA private key and digital certificate
+
+1. Install Openssl
+
+Please visit https://github.com/openssl/openssl to get pkg and install.
+
+2. Generate RSA private key
+
+```sh
+$ mkdir testdata
+$ openssl genrsa -out ./testdata/server.key 2048
+```
+
+3. Generate digital certificate
+
+```sh
+$ openssl req -new -x509 -key ./testdata/server.key -out ./testdata/server.pem -days 365
+```