suggested addition to documentation for building PDFs on readthedocs.io

[dsavransky]

Thanks for an incredibly useful project! Minor suggested addition to the README and/or docs:

In order to get mermaid to run properly when generating PDFs with automated builds in readthedocs.io:

1. In your .readthedocs.yaml file have something like:

build:
  os: ubuntu-20.04
  tools:
    python: "3.8"
    nodejs: "16"
  jobs:
    post_install:
      - npm install -g @mermaid-js/mermaid-cli

2. In your documentation directory add file puppeteer-config.json with contents:

{
 "args": ["--no-sandbox"]
}

3. In conf.py add:

mermaid_params = ['-p' 'puppeteer-config.json']

[mgaitan]

@dsavransky thank you for your words.

could you send it as a PR changing as a new section of the docs and maybe a link from the readme ? Also feel free to change the configs so we get our own docs in PDF format.