Updated README.md per discussion here http://disq.us/p/298qamr . Note that I...

Updated README.md per discussion here http://disq.us/p/298qamr . Note that I used an absolute link to the GitLab runner documentation, which seems to be cross-repo, so I didn't think it would work to use the filename.md type path link.

doc/ci/variables/README.md

@@ -487,8 +487,10 @@ variables take precedence over those defined in `.gitlab-ci.yml`.
 
 ## Unsupported variables
 
-There are cases where some variables cannot be used in the context of a
-`.gitlab-ci.yml` definition (for example under `script`). Read more about which variables are [not supported](where_variables_can_be_used.md).
+Variable names are limited by the underlying shell used to execute scripts (see [available shells](https://docs.gitlab.com/runner/shells/index.html).
+Each shell has its own unique set of reserved variable names.
+You will also want to keep in mind the [scope of environment variables](where_variables_can_be_used.md) to ensure a variable is defined in the scope
+in which you wish to use it.
 
 ## Where variables can be used