Add a vignette

[gregorgorjanc]

See https://github.com/RcppCore/RcppArmadillo/tree/master/vignettes

Perhaps on showing how to use tskit C API via Rcpp (in C++ code)- this example shows how to build ts tables https://github.com/tskit-dev/tskit-build-examples/blob/main/example.c

[gregorgorjanc]

use_vignette()
Create a vignette
https://usethis.r-lib.org/reference/use_vignette.html

Use contents in https://github.com/HighlanderLab/tskitr/blob/main/tskitr/inst/STATE_and_AIMS.md

[gregorgorjanc]

Closing as fixed in #43