add article that explains lower/upper case for (un-)masked

[marehr]

Tasks

• Find and add an article that explains the use case for lower/upper case alphabets
• add that article to seqan3::masked too

From 29c2f4894f0d619b792532e3dcb78c0562281d0f Mon Sep 17 00:00:00 2001
From: marehr <marehr-github@marehr.dialup.fu-berlin.de>
Date: Wed, 14 Apr 2021 11:04:21 +0200
Subject: [PATCH 04/12] TODO: [DOC] add article that explains lower/upper case
 for (un-)masked

---
 include/seqan3/alphabet/mask/mask.hpp | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/seqan3/alphabet/mask/mask.hpp b/include/seqan3/alphabet/mask/mask.hpp
index 9001e6237..ef7ffb164 100644
--- a/include/seqan3/alphabet/mask/mask.hpp
+++ b/include/seqan3/alphabet/mask/mask.hpp
@@ -30,6 +30,7 @@ namespace seqan3
  *
  * \stableapi{Since version 3.1.}
  */
+// TODO add article which explains lower and upper
 class mask : public alphabet_base<mask, 2, void>
 {
 private:
-- 
2.31.1