Make CHANGELOG markdown

Explicitly mark the CHANGELOG as being markdown.

Author: martincostello

CHANGELOG CHANGELOG.md

@@ -10,7 +10,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 		.gitignore = .gitignore
 		.vsconfig = .vsconfig
 		build.ps1 = build.ps1
-		CHANGELOG = CHANGELOG
+		CHANGELOG.md = CHANGELOG.md
 		CODE_OF_CONDUCT.md = CODE_OF_CONDUCT.md
 		Directory.Build.props = Directory.Build.props
 		Directory.Build.targets = Directory.Build.targets