providing a more direct link for R documentation to writeMat within the README

while reading the documentation, i found that this link could streamline the implementation process by taking the reader directly to the usage documentation for the writeMat function and it's arguments. I tried to make it so that you would only have to copy and paste the old link that was in parenthesis.

"(https://cran.r-project.org/web/packages/R.matlab/R.matlab.pdf -> follow the page number link within the table of contents to the writeMat function documentation for argument definitions.)"