title: Library is not writable
body: <p>I have this issue during package installation in R version 3.0.2 (2013-09-25) on an Ubuntu machine:</p>
  
  <pre><code>install.packages("randomForest")
  Installing package into ‘/usr/local/lib/R/site-library’
  (as ‘lib’ is unspecified)
  Warning in install.packages :
    'lib = "/usr/local/lib/R/site-library"' is not writable
  </code></pre>
  
  <p>How to resolve it?</p>