Add note for how to opt into old behavior to changelog

Signed-off-by: Jesse Szwedko <[email protected]>

changelog.d/deprecate_strict_env_vars.breaking.md

@@ -1,4 +1,4 @@
 The default of `--strict-env-vars` has been changed to `true`. This option has been deprecated. In
 a future version it will be removed and Vector will have the behavior it currently has when set
 to `true` which is that missing environment variables will cause Vector to fail to start up with an
-error instead of a warning.
+error instead of a warning. Set `--strict-env-vars=false` to opt into deprecated behavior.