Adding `.` to list of masked variables

doc/ci/variables/README.md

@@ -209,9 +209,10 @@ The value of the variable must:
 - Be in a single line.
 - Be at least 8 characters long.
 - Not be a predefined or custom environment variable.
-- Consist only of characters from the Base64 alphabet (RFC4648).
-  [In GitLab 12.2](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/63043)
-  and newer, `@` and `:` are also valid values.
+- Consist only of:
+  - Characters from the Base64 alphabet (RFC4648).
+  - The `@` and `:` characters ([In GitLab 12.2](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/63043) and later).
+  - The `.` character ([In GitLab 12.10](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/29022) and later).
 
 You can't mask variables that don't meet these requirements.