release: Add relase notes to /docs for release of 0.20.5

[ci skip]

Author: bep

docs/content/release-notes/0.20.5-relnotes.md

@@ -0,0 +1,9 @@
+
+---
+date: 2017-04-25
+title: 0.20.5
+---
+
+	
+
+This is a bug-fix relase which fixes the version number of `0.20.4` (which wrongly shows up as `0.21-DEV`) ([#3388](https://github.com/spf13/hugo/issues/3388)).