Add ccache installation and setup instructions

[bobjansen]

As a reaction to a question raised in the audience during userR! and the
discussion on #103, an extra section in the README that explains
(hopefully clearly) how to setup ccache for use with R and pak to speed
up installation of packages that need compilation.

Credits to @eddelbuettel for making me aware of this idea. Settings taken
from his blog at

http://dirk.eddelbuettel.com/blog/2017/11/27/

Compared to the post I've reduced maximum cache size from 5G to 1G as
installation of packages tidyverse, devtools and data.table generates
less than 150M of cache files and extended the inline comments
somewhat.

[gaborcsardi]

Thanks! A pretty good write up, too good. :) I think it is too much for the pak README, which is pretty short currently, and tries to focus on the package itself, but I think we can find another place for it.

[bobjansen]

I agree it is too long. I could put it in a vignette and refer to it from the README?

[jimhester]

FWIW if you install ccache with homebrew it sets up symlinks for all the compiler aliases during installation, so you can set your PATH so that $BREWPREFIX/opt/ccache/libexec comes first and use ccache for everything without messing with Makevars at all. This is what I do.