Rename release note files to CHANGELOG.md and CHANGELOG_ja.md.

Author: hymkor

release_note_en.md CHANGELOG.mdrelease_note_en.md renamed to CHANGELOG.md

@@ -1,6 +1,6 @@
-Release notes (English)
+Changelog (English)
 =======================
-( **English** / [Japanese](release_note_ja.md) )
+( **English** / [Japanese](CHANGELOG_ja.md) )
 
 - Fixed an issue where the version string was empty when built without GNU Make.  
   The version string is now updated via make bump during the release process. (#46)

release_note_ja.md CHANGELOG_ja.mdrelease_note_ja.md renamed to CHANGELOG_ja.md

@@ -1,6 +1,6 @@
-Release notes (Japanese)
+Changelog (Japanese)
 ========================
-( [English](release_note_en.md) / **Japanese** )
+( [English](CHANGELOG.md) / **Japanese** )
 
 - GNU Make なしでビルドした場合に、バージョン文字列が空になってしまう問題を修正  
   今後、バージョンアップ時に make bump を実行する (#46)