Update read me to be copy/pastable runnable

[jakebruemmer]

I was looking through this repo and decided to run the code listed in the README after installing the gem. I got an error:

2.5.0 :001 > require 'metro_2'
 => true 
2.5.0 :002 > metro_2_content = Metro2::Metro2File.new
metro_2_content.header.cycle_number = 15
metro_2_content.header.equifax_program_identifier = 'EFAXID'
metro_2_content.header.transunion_program_identifier = 'TRANSUNION'
# ... other header segment attributes

base_segment = Metro2::Records::BaseSegment.new
base_segment.time_stamp = Time.new(2014, 9, 15, 17, 7, 45)
base_segment.identification_number = 'REPORTERXYZ'
base_segment.cycle_number = 1
base_segment.consumer_account_number = 'ABC123'
base_segment.portfolio_type = 'I'
# ... other base segment attributes
metro_2_content.base_segments << base_segment
# add more base segments as needed

metro_2_content.to_s # contents to write to file => #<Metro2::Metro2File:0x00007fd7df8adce8 @base_segments=[], @header=#<Metro2::Records::HeaderSegment:0x00007fd7df8adc98>> 
2.5.0 :003 >  => 15 
2.5.0 :004 >  => "EFAXID" 
2.5.0 :005 >  => "TRANSUNION" 
2.5.0 :006 >  => nil 
2.5.0 :007 > 2.5.0 :008 >  => #<Metro2::Records::BaseSegment:0x00007fd7df84b6d8 @appendages=[]> 
2.5.0 :009 >  => 2014-09-15 17:07:45 -0500 
2.5.0 :010 >  => "REPORTERXYZ" 
2.5.0 :011 >  => 1 
2.5.0 :012 >  => "ABC123" 
2.5.0 :013 >  => "I" 
2.5.0 :014 >  => nil 
2.5.0 :015 >  => [#<Metro2::Records::BaseSegment:0x00007fd7df84b6d8 @appendages=[], @time_stamp=2014-09-15 17:07:45 -0500, @identification_number="REPORTERXYZ", @cycle_number=1, @consumer_account_number="ABC123", @portfolio_type="I">] 
2.5.0 :016 >  => nil 
2.5.0 :017 > 2.5.0 :018 > exit
Traceback (most recent call last):
        6: from /Users/jake/.rvm/rubies/ruby-2.5.0/bin/irb:11:in `<main>'
        5: from (irb):18
        4: from /Users/jake/.rvm/gems/ruby-2.5.0/gems/metro_2-1.2.4/lib/metro_2/metro2_file.rb:20:in `to_s'
        3: from /Users/jake/.rvm/gems/ruby-2.5.0/gems/metro_2-1.2.4/lib/metro_2/metro2_file.rb:34:in `trailer_from_base_segments'
        2: from /Users/jake/.rvm/gems/ruby-2.5.0/gems/metro_2-1.2.4/lib/metro_2/metro2_file.rb:34:in `each'
        1: from /Users/jake/.rvm/gems/ruby-2.5.0/gems/metro_2-1.2.4/lib/metro_2/metro2_file.rb:35:in `block in trailer_from_base_segments'
NoMethodError (undefined method `upcase' for nil:NilClass)

which is from base_segment.account_status being nil since it's not set in the code. This PR adds a line to the README with an account_status for a current loan. The result:

2.5.0 :001 > require 'metro_2'
 => true 
2.5.0 :002 > metro_2_content = Metro2::Metro2File.new
metro_2_content.header.cycle_number = 15
metro_2_content.header.equifax_program_identifier = 'EFAXID'
metro_2_content.header.transunion_program_identifier = 'TRANSUNION'
# ... other header segment attributes

base_segment = Metro2::Records::BaseSegment.new
base_segment.time_stamp = Time.new(2014, 9, 15, 17, 7, 45)
base_segment.identification_number = 'REPORTERXYZ'
base_segment.cycle_number = 1
base_segment.consumer_account_number = 'ABC123'
base_segment.portfolio_type = 'I'
base_segment.account_status = '11'
# ... other base segment attributes
metro_2_content.base_segments << base_segment
# add more base segments as needed

metro_2_content.to_s # contents to write to file => #<Metro2::Metro2File:0x00007fa9991aaf50 @base_segments=[], @header=#<Metro2::Records::HeaderSegment:0x00007fa9991aaf00>> 
2.5.0 :003 >  => 15 
2.5.0 :004 >  => "EFAXID" 
2.5.0 :005 >  => "TRANSUNION" 
2.5.0 :006 >  => nil 
2.5.0 :007 > 2.5.0 :008 >  => #<Metro2::Records::BaseSegment:0x00007fa999162980 @appendages=[]> 
2.5.0 :009 >  => 2014-09-15 17:07:45 -0500 
2.5.0 :010 >  => "REPORTERXYZ" 
2.5.0 :011 >  => 1 
2.5.0 :012 >  => "ABC123" 
2.5.0 :013 >  => "I" 
2.5.0 :014 >  => "11" 
2.5.0 :015 >  => nil 
2.5.0 :016 >  => [#<Metro2::Records::BaseSegment:0x00007fa999162980 @appendages=[], @time_stamp=2014-09-15 17:07:45 -0500, @identification_number="REPORTERXYZ", @cycle_number=1, @consumer_account_number="ABC123", @portfolio_type="I", @account_status="11">] 
2.5.0 :017 >  => nil 
2.5.0 :018 > 2.5.0 :019 > 
 => "0426HEADER15          EFAXID         TRANSUNION00000000000000000000000000000000                                                                                                                                        0000000000Upstart Engineer metro 2 gem            01204                                                                                                                                                            \n04261091520141707450REPORTERXYZ         1 ABC123                        I  00000000000000000000000000    00000000000000000011                             00000000000000000000000000000000000000000000000000000000000                                                                                    000000000000000000000000000                                                                                                      \n0426TRAILER000000001         000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000                   \n"