Clean build

[KevCaz]

This PR solves #19 and #18. Plus, I've slightly re-organized the code as follows so that helper functions are all in zzz.R, and the functions actually exposed in esri2sf.R. This is just a way to work that help me work on this, it is totally fine to put all function in the same file. Last, I turned the readme into a Rmd files so it gives a simple visual way to check that the code is actually working fine, the following line show how to use it.

library(rmakdown)
render("README.Rmd", "all")

Now the package can be built smoothly:

── R CMD check results ────────────── esri2sf 0.1.1 ────
Duration: 1m 5.5s

0 errors ✔ | 0 warnings ✔ | 0 notes ✔