Update README.md

Clarify the difference between the usages of Roo::Spreadsheet.open vs Roo::Excelx.new

README.md

@@ -22,22 +22,37 @@ gem "roo", "~> 2.10.0"
 ```
 ## Usage
 
-Opening a spreadsheet
+### Opening a spreadsheet
 
+You can use the `Roo::Spreadsheet` class so `roo` automatically detects which [parser class](https://github.com/roo-rb/roo/blob/master/lib/roo.rb#L17) to use for you.
 ```ruby
 require 'roo'
 
-xlsx = Roo::Spreadsheet.open('./new_prices.xlsx')
-xlsx = Roo::Excelx.new("./new_prices.xlsx")
+file_name = './new_prices.xlsx'
+xlsx = Roo::Spreadsheet.open(file_name)
+xlsx.info
+# => Returns basic info about the spreadsheet file
+```
 
-# Use the extension option if the extension is ambiguous.
-xlsx = Roo::Spreadsheet.open('./rails_temp_upload', extension: :xlsx)
+``Roo::Spreadsheet.open`` can accept both string paths and ``File`` instances. Also, you can provide the extension of the file as an option: 
+
+```ruby
+require 'roo'
 
+file_name = './rails_temp_upload'
+xlsx = Roo::Spreadsheet.open(file_name, extension: :xlsx)
 xlsx.info
 # => Returns basic info about the spreadsheet file
 ```
 
-``Roo::Spreadsheet.open`` can accept both paths and ``File`` instances.
+On the other hand, if you know what the file extension is, you can use the specific parser class instead:
+```ruby
+require 'roo'
+
+xlsx = Roo::Excelx.new("./new_prices.xlsx")
+xlsx.info
+# => Returns basic info about the spreadsheet file
+```
 
 ### Working with sheets