Add reference changes to UPGRADING documentation.

UPGRADING.md

@@ -115,6 +115,8 @@
     - `Psalm\Type\Union`
  - While not a BC break per se, all classes / interfaces / traits / enums under
    `Psalm\Internal` namespace are now marked `@internal`.
+ - To remove a variable from the context, Context::remove(). Calling
+   `unset($context->vars_in_scope[$var_id])` can cause problems when using references.
 
 ## Removed
  - [BC] Property `Psalm\Codebase::$php_major_version` was removed, use
@@ -156,5 +158,5 @@
  - [BC] Method `Psalm\Issue\CodeIssue::getMessage()` was removed
  - [BC] Method `Psalm\DocComment::parse()` was removed
  - [BC] Class `Psalm\Type\Atomic\THtmlEscapedString` has been removed
-
+ - [BC] Property `Psalm\Context::$vars_from_global` has been removed