Update README

[ThierryO]

As suggested by ropensci/software-review#263 (comment)

• Elaborate on the rationale of git2rdata
• Add a link to the contributing guidelines
• Update installation instructions using remotes instead of devtools and add option to build vignettes
• Add a link to the vignettes
• add ROpenSci review badge
• move lifecycle badge from "experimental" to "maturing"
• move status badge from "WIP" to "Active"
• add short summary of the vignettes to the README
• add some rough guidelines about maximal size of the data