Update ISSUE_TEMPLATE

Author: sasurau4

.github/ISSUE_TEMPLATE/bug_report.md

@@ -14,6 +14,7 @@ Forum for questions: https://users.rust-lang.org/c/ide/14
 Before submitting, please make sure that you're not running into one of these known issues:
 
  1. on-the-fly diagnostics are mostly unimplemented (`cargo check` diagnostics will be shown when saving a file)
+ 2. attribute proc macros are supported but not enabled by default: #6029
 
 Otherwise please try to provide information which will help us to fix the issue faster. Minimal reproducible examples with few dependencies are especially lovely <3.
 -->