Add .crt support to docs
Add .crt support to docs

Docs for https://github.com/boot2docker/boot2docker/pull/1167
SvenDowideit/boot2docker,tianon/boot2docker-legacy,whuwxl/boot2docker,boot2docker/boot2docker,power-dev-env/boot2docker,SvenDowideit/boot2docker,whuwxl/boot2docker,qxo/boot2docker,qxo/boot2docker,power-dev-env/boot2docker