title: How to do group by count in R
body: <p>I want to have count of booking IDs on Month-Source Level</p>
  
  <pre><code>Month   Source  Booking_id
  Oct        A    100
  Nov        B    101
  Oct        A    106
  Jan        B    109
  Nov        A    110
  Nov        B    111
  
  
  data &lt;- structure(list(Month = c("October", "November", "October", "January", 
  "November", "November"), Source = c("A", "B", "A", "B", "A", 
  "B"), Booking_ID = c(100L, 101L, 106L, 109L, 110L, 111L)), .Names = c("Month", 
  "Source", "Booking_ID"), class = c("tbl_df", "tbl", "data.frame"
  ), row.names = c(NA, -6L))
  </code></pre>