Completed the first task of about_arrays.rb

* Pivotal Traker Id [#75411406]
* learned that an empty array is still in the class Array
* learend that the size of an empty array is 0
Completed task 2 of about_arrays.rb

* pivotal tracker id [#75411406]
* learned about array literals
* remembered that the << operator appends an element to an array. Horray!
completed task 3 of about_arrays.rb

* pivotal tracker id {#75411406}
* learned about accesing array elements by index
* arrays have a 0 based index
* negative indices count backwards from the end of the array