Update Description to note `systemRequirements`

[setgree]

For kicks, I ran

docker run --rm -it rocker/r-base:3.6.3
install.packages('remotes')
remotes::install_github('karthik/holepunch')

And I got the following errors about missing system-level dependencies:

------------------------- ANTICONF ERROR ---------------------------
Configuration failed because libcurl was not found. Try installing:
 * deb: libcurl4-openssl-dev (Debian, Ubuntu, etc)
 * rpm: libcurl-devel (Fedora, CentOS, RHEL)
 * csw: libcurl_dev (Solaris)

and

------------------------- ANTICONF ERROR ---------------------------
Configuration failed because libxml-2.0 was not found. Try installing:
 * deb: libxml2-dev (Debian, Ubuntu, etc)
 * rpm: libxml2-devel (Fedora, CentOS, RHEL)
 * csw: libxml2_dev (Solaris)

This won't be strictly sufficient for a new user -- R doesn't have a system for automating the systemRequirements checks -- but it's a good start, and if efforts like https://github.com/r-hub/sysreqsdb take off, it might eventually suffice.

Thanks either way for a great package!