Add a project spec for future changelog entry

CHANGELOG.md is already mixed with and without URLs,
so direct editing of CHANGELOG.md was hard to prevent formatting without URL,
but the new form of adding future changelog entry to the changelog directory can force URL.

Instead of reviewer pointing out, the automated test failure can inform that to reviewee.

CONTRIBUTING.md

@@ -93,7 +93,6 @@ Here are a few examples:
 
 ```markdown
 * [#716](https://github.com/rubocop/rubocop/issues/716): Fixed a regression in the autocorrection logic of `MethodDefParentheses`. ([@bbatsov][])
-* New cop `ElseLayout` checks for odd arrangement of code in the `else` branch of a conditional expression. ([@bbatsov][])
 * [#7542](https://github.com/rubocop/rubocop/pull/7542): **(Breaking)** Move `LineLength` cop from `Metrics` department to `Layout` department. ([@koic][])
 ```
 

spec/project_spec.rb

@@ -242,6 +242,13 @@ def expected
 
           include_examples 'has Changelog format'
 
+          it 'has a link to the issue or pull request address at the beginning' do
+            repo = 'rubocop/rubocop'
+            address_pattern = %r{\A\* \[#\d+\]\(https://github\.com/#{repo}/(issues|pull)/\d+\):}
+
+            expect(entries).to all(match(address_pattern))
+          end
+
           it 'has a link to the contributors at the end' do
             expect(entries).to all(match(/\(\[@\S+\]\[\](?:, \[@\S+\]\[\])*\)$/))
           end