title: Make a file writable in order to add new packages
body: <p>I use Rstudio and try to add new packages through command console.</p>
  
  <p>It fails to install it for this reason</p>
  
  <pre><code>install.packages("devtools"); devtools::install_github("hadley/rvest")
  Warning in install.packages :
    'lib = "C:/Program Files/R/R-3.2.1/library"' is not writable
  Warning in install.packages :
    cannot create dir 'C:\Users\myuser', reason 'Permission denied'
  Error in install.packages : unable to create ‘C:/Users/myuser/Documents/R/win-library/3.2’
  Error in loadNamespace(name) : there is no package called ‘devtools’
  </code></pre>
  
  <p>How can I make this file writable?</p>