Improve README

[rossant]

• make it shorter
• put the installation instructions at the beginning
• document the CLI conversion tool

[mathcass]

I can echo a desire for more examples in the README. Through trial and error, I figured out how to use the CLI conversion tool. I tried these incorrect examples:

ipymd POST.ipynb --from ipynb --to md

ipymd POST.ipynb --from .ipynb --to .md

only for this correct one to work:

ipymd POST.ipynb --from notebook --to markdown

It wasn't clear to me from the README what the valid values were for the to and from arguments.

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BTW, thanks for the neat tool.

[rossant]

100% agree, PR welcome! Thanks.