Add clarification about how to contribute to README

[vorburger]

I wanted to make a (small) edit to https://beancount.github.io/docs/external_contributions.html (to add the full list of the existing importers of @tarioch https://github.com/tarioch/beancounttools), and figured this Git repo is the source of that site.

It's just not clear (to me) yet, neither from the README nor the CONTRIBUTING, how one may raise a PR or suggest an edit - in whatever form the maintainers of this (amazing) community prefer.

CONTRIBUTING says to check this thread which I have glanced over but still not understood how to.

Perhaps this could be clarified; I'm happy to raise a PR if when I learn how you like contribs.

@bradyt @xuhcc

[xuhcc]

@vorburger In the past contributing required write access to google documents. Some community members had it, but things could have changed.
I think it's better to ask about that in the maling list.