Add Iterate With An Offset Index as a ruby til

ruby/iterate-with-an-offset-index.md

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+# Iterate With An Offset Index
+
+You can iterate over a collection of items with the
+[`#each`](https://devdocs.io/ruby~2.5/enumerator#method-i-each) method. If you
+want to know the index of each item as you go, you can use the
+[`#each_with_index`](https://devdocs.io/ruby~2.5/enumerable#method-i-each_with_index)
+variant.
+
+```ruby
+> ["one", "two", "three"].each_with_index do |item, index|
+    puts "#{item} - #{index}"
+  end
+one - 0
+two - 1
+three - 2
+=> ["one", "two", "three"]
+```
+
+The initial index will always be `0` when using `#each_with_index`.
+
+What about if you want the index value to be offset by some number?
+
+You can use the
+[`#with_index`](https://devdocs.io/ruby~2.5/enumerator#method-i-with_index)
+method on an _enumerator_. It optionally takes an `offset` argument.
+
+```ruby
+> ["one", "two", "three"].each.with_index(1) do |item, index|
+    puts "#{item} - #{index}"
+  end
+one - 1
+two - 2
+three - 3
+=> ["one", "two", "three"]
+```