Consider adding a link to releases in README.md

[MikkoVihtakari]

As more people are attempting to install and use the new Rstox packages, there seem to be more frequent questions concerning problems with installing the packages. The majority of these problems are related to Rtools failing to compile C++ code under institutional Windows machines. These emails could be reduced by adding instructions on installing binary for each of the StoXProject packages. E.g:

On computers that return errors when trying to run the Rtools through RStudio (most institutional Windows machines), install the binary directly from [here] (<link to releases, e.g. https://github.com/StoXProject/RstoxData/releases>). Download the newest RstoxData zip file, click the “Packages” tab -> “Install” -> “Install from:” “Package Archive File” -> “Install”. If the installer does not complain, the package is installed correctly.