doc for subscribe support

[ikq]

It's documentation for SUBSCRIBE/NOTIFY support.

It passed nanoc compile.

P.S.
There was nanoc installation problem, so I use a docker image with nanoc:

• extends existed Docker image

$ cat Dockerfile
FROM jovandeginste/nanoc
RUN echo "gem 'RedCloth'" >> /Gemfile
RUN bundle install

$docker build -t mynanoc .

• Run the image:

$docker run -it -w /data --entrypoint /bin/bash -v ./jssipdoc:/data mynanoc

• Run within docker container image:
  bundle exec nanoc compile

[jmillan]

Thanks @ikq!

I'll review this after Christmas 👍