Update README to explain Rtools is required for package install and signpost to install guide

[oliverodolin]

Confirm whether assertHE requires Rtools for a successful install. If so, update the README to explain this requirement.

@RobertASmith This may not be necessary if we change the version requirements for some of the dependencies to be lower.

[JettingerParexel]

Hey Oliver. I have downloaded the most recent version of RTools and didn't seem to change the error I was getting.