readme: add an example output
```
$ sudo h2olog -p $(pgrep -o h2o)
+
+888 1: HTTP/2.0
+888 1: :authority torumk.com
+888 1: :method GET
+888 1: :path /
+888 1: :scheme https
+... and more ...
```
Root privilege is required to interact with the BPF virtual machine.
