72 characters per line for .md in .editorconfig

Summary:
72 characters is optimal for text reading, because the optimal number of letters per line is about 60, which makes about 72 characters with spaces.

Note IETF RFC uses 72 characters ([example](https://datatracker.ietf.org/doc/html/rfc9113)).

Reviewed By: iguridi

Differential Revision: D46652047

fbshipit-source-id: 1f506b64070d9a80555dae1c77313183ebe0396c

.editorconfig

@@ -8,6 +8,9 @@ max_line_length = 100
 indent_size = 4
 tab_width = 4
 
+[*.md]
+max_line_length = 72
+
 [.patternlint/config.json]
 indent_size = 2
 tab_width = 2