Update README of R-targeted GitHub repository

[benesom]

Repo writes

but on CRAN we have]

Various methods for targeted and semiparametric inference including augmented inverse probability weighted (AIPW) estimators for missing data and causal inference (Bang and Robins (2005) <doi:10.1111/j.1541-0420.2005.00377.x>), variable importance and conditional average treatment effects (CATE) (van der Laan (2006) <doi:10.2202/1557-4679.1008>), estimators for risk differences and relative risks (Richardson et al. (2017) <doi:10.1080/01621459.2016.1192546>), assumption lean inference for generalized linear model parameters (Vansteelandt et al. (2022) <doi:10.1111/rssb.12504>).

[kkholst]

The README definitely needs an update. We should also reference or link to the CONTRIBUTING document.
I also suggest we create new examples that briefly shows some of the main functionality, e.g. the cate function.