crypto/tls: add examples for [Load]X509KeyPair

I was confused about how to start an HTTP server if the server
cert/key are in memory, not on disk. I thought it would be good to
show an example of how to use these two functions to accomplish that.

example-cert.pem and example-key.pem were generated using
crypto/tls/generate_cert.go.

Change-Id: I850e1282fb1c38aff8bd9aeb51988d21fe307584
Reviewed-on: https://go-review.googlesource.com/72252


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>