Add nightly proc macro issue to the bug report template

Author: lnicola

.github/ISSUE_TEMPLATE/bug_report.md

@@ -13,8 +13,9 @@ 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)
+ 1. on-the-fly diagnostics are mostly unimplemented (`cargo check` diagnostics will be shown when saving a file): #3107
  2. attribute proc macros are supported but not enabled by default: #6029
+ 3. proc macros crash the language server on nightly and beta compiler versions: #8925
 
 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.
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