Add more detail to the README

[TomKellyGenetics]

I've had some questions about how to use Git/GitHub to contribute. Especially for other lessons such as R, they might not know how to send a pull request. Would it be possible to include more detail in the README on how to clone a repo or add a submodule.

I've added some detail on the Japanese repo/branch here but it mainly concerns contributing to a translation in progress (and still assumes basic knowledge of git):

https://github.com/swcarpentry-ja/i18n

[TomKellyGenetics]

The Japanese repo now has more details. swcarpentry-ja#6

More detail has been added so that users with less experience with Git can contribute to translations. How to merge updated lessons still needs more consideration.

[dpshelio]

Since more tooling is now involved, like transifex, I believe it would be better to create a repo with a sphinx project, like Handbook for translators. So that it could be also translated too 😄

[dpshelio]

Repo created! Introducing the i18n-handbook