Streamline installation instructions

[karawoo]

This PR makes a small change to the DESCRIPTION file so that augsynth will be installed automatically, with no need for a separate call to install_github(). With Remotes: ebenmichael/augsynth in the DESCRIPTION file, remotes will know to install augsynth from GitHub when installing GeoLift. Reproducible example below showing that it works:

remove.packages("augsynth")
#> Removing package from '/Users/kara/Library/R/arm64/4.4/library'
#> (as 'lib' is unspecified)

"augsynth" %in% installed.packages()
#> [1] FALSE

remotes::install_github("karawoo/GeoLift@streamline-installation", force = TRUE, quiet = TRUE)
c("augsynth", "GeoLift") %in% installed.packages()
#> [1] TRUE TRUE

^{Created on 2024-05-28 with reprex v2.1.0}

Hope you'll pardon my opening a PR without filing an issue first; if there's a specific reason for installing augsynth separately then please disregard this PR.