Document how to format localized dates in Ruby

This documents the two ways we currently localized dates:
1) Using l helper
2) Through strftime

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41237

doc/development/i18n/externalization.md

@@ -228,6 +228,16 @@ This makes use of [`Intl.DateTimeFormat`].
 
 [`Intl.DateTimeFormat`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat
 
+- In Ruby/HAML, we have two ways of adding format to dates and times:
+
+  1. **Through the `l` helper**, i.e. `l(active_session.created_at, format: :short)`. We have some predefined formats for
+[dates](https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.7.0/config/locales/en.yml#L54) and [times](https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.7.0/config/locales/en.yml#L261).
+  If you need to add a new format, because other parts of the code could benefit from it,
+  you'll need to add it to [en.yml](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/locales/en.yml) file.
+  2. **Through `strftime`**, i.e. `milestone.start_date.strftime('%b %-d')`. We use `strftime` in case none of the formats
+  defined on [en.yml](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/locales/en.yml) matches the date/time 
+  specifications we need, and if there is no need to add it as a new format because is very particular (i.e. it's only used in a single view).
+
 ## Best practices
 
 ### Splitting sentences