Add note about nested variables for GIT_CLONE_PATH

The variable $GIT_CLONE_PATH does not support nested variables, it will
only expand the variables once, not more.

reference https://gitlab.com/gitlab-org/gitlab-runner/issues/4319

doc/ci/yaml/README.md

@@ -2620,6 +2620,24 @@ test:
     - pwd
 ```
 
+### Nested paths
+
+The value of `GIT_CLONE_PATH` is expanded once and nesting variables
+within it is not supported.
+
+For example, you define both the variables below in your
+`.gitlab-ci.yml` file:
+
+```yml
+variables:
+  GOPATH: $CI_BUILDS_DIR/go
+  GIT_CLONE_PATH: $GOPATH/src/namespace/project
+```
+
+The value of `GIT_CLONE_PATH` is expanded once into
+`$CI_BUILDS_DIR/go/src/namespace/project`, and results in failure
+because `$CI_BUILDS_DIR` is not expanded.   
+
 ## Special YAML features
 
 It's possible to use special YAML features like anchors (`&`), aliases (`*`)