Document how to run with SSL, and how to fully enable HTTP2 support in the builtin webserver

[fredemmott]

hhvm -m server \
  -p 8080 \
  -v Server.SSLPort=8443 \
  -v Server.EnableSSL=true \
  -v Server.SSLCertificateFile=server.crt \
  -v Server.SSLCertificateKeyFile=server.key \
  -v Server.EnableH2C=true \
  -v Server.SSLNextProtocols.0=h2

H2C is HTTP2 without SSL

[afrind]

h2 specification for h2c: https://tools.ietf.org/html/rfc7540#section-8.1.1