Move/copy part of the README to a get started vignette

[maelle]

Currently the optiRum title in articles in the website refers to the articles page itself.

[maelle]

@stephlocke do you want to keep the summary of functions in the README? I imagine so. In that case, before the release I need to check it's up-to-date (unless I spend time finding a way to automatically reproduce https://itsalocke.com/optirum/reference/ in the README 😉).

[stephlocke]

Put the Rmd back and I imagine you can ;)

[maelle]

That or a fancy tic setup 🤔 with a script generating the Markdown, directly put into the README.md, from Travis.