Add Fetch Warns About Superseding Block Argument as a Ruby TIL

ruby/fetch-warns-about-superseding-block-argument.md

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+# Fetch Warns About Superseding Block Argument
+
+[Ruby's `#fetch`](https://ruby-doc.org/core-2.5.1/Hash.html#method-i-fetch) can
+be used in a couple ways beyond just grabbing the value out of a hash.
+
+If you include a second argument in the `#fetch` call, that will be treated as
+a default value to fallback to when the first argument key doesn't appear in
+the hash.
+
+If you instead specify a block argument, that block will be executed when the
+key is missing.
+
+What happens when you specify both a second argument and a block argument?
+
+```ruby
+data = { taco: 'bell' }
+
+data.fetch(:burrito, 'house') do
+  puts 'the block gets executed'
+  'shack'
+end
+
+warning: block supersedes default value argument
+the block gets executed
+=> 'shack'
+```
+
+The block argument wins. The second argument is ignored. And Ruby warns you
+that, "block supersedes default value argument".