(#495) Add docs for new configuration option

Add information to the docs section for the new allow-milestone-without-issues option.

Author: gep13

docs/input/docs/configuration/default-configuration.md

@@ -21,6 +21,7 @@ create:
  sha-section-line-format: "- `{1}\t{0}`"
  allow-update-to-published: false
  include-contributors: false
+ allow-milestone-without-issues: false
 export:
  include-created-date-in-title: false
  created-date-string-format: ''
@@ -142,6 +143,11 @@ control the look and feel of the generated release notes.
  in the release notes. A contributor is defined as someone who opened an issue
  or submitted a PR. **NOTE:** This configuration option was added in version
  0.19.0 of GitReleaseManager.
+- **allow-milestone-without-issues**
+ - A boolean value which indicates whether an empty release will be created, when
+ no issues are found to be associated with a milestone. The contents of the
+ empty release can be controlled via the associated Scriban template.
+ **NOTE:** This configuration option was added in version 0.20.0 of GitReleaseManager.
 
 See the [example create configuration section](create-configuration) to see an
 example of how a footer can be configured.