ADDITIONAL_MODULE_TABS Ignores RELEASE_NOTES.md

Description: I noticed that when using the ADDITIONAL_MODULE_TABS functionality in Terrareg, the content of CHANGELOG.md is correctly displayed in the UI under the Release Notes tab as expected. However, despite RELEASE_NOTES.md being a supported file by default according to the documentation, its content is not presented in the UI. This appears to be an inconsistency in how these files are being handled by the ADDITIONAL_MODULE_TABS feature.

Steps to Reproduce: Add CHANGELOG.md and RELEASE_NOTES.md to the module folder. Observe that only the content of CHANGELOG.md is displayed in the UI.

Expected Behavior: The content of both CHANGELOG.md and RELEASE_NOTES.md should be displayed in the UI.

Github reference: https://github.com/MatthewJohn/terrareg/issues/60