add steps to install Rtools for {EpiNow2} to compile in Windows

[avallecam]

When installing from CRAN, a user experience ERROR: compilation failed for package 'EpiNow2'

Windows users will need a working installation of Rtools in order to build the package from source.

Add steps from http://jtleek.com/modules/01_DataScientistToolbox/02_10_rtools/#1

1. download the Rtools installer from https://cran.r-project.org/bin/windows/Rtools/
2. install Rtools with default selections
3. install {devtools} -->  install.packages("devtools")
4. verify Rtools installation with --> devtools::find_rtools()

To fix in #61