Add StackOverflow link to README (#4694)

.github/ISSUE_TEMPLATE/bug_report.md

@@ -10,6 +10,8 @@ assignees: ''
 <!--
 Please fill this template entirely and do not erase any of it.
 We reserve the right to close without a response bug reports which are incomplete.
+
+If you have a question rather than a bug, please ask on [Stack Overflow](https://stackoverflow.com/questions/tagged/allennlp) rather than posting an issue here.
 -->
 
 ## Checklist

.github/ISSUE_TEMPLATE/question.md

@@ -0,0 +1,10 @@
+---
+name: Question
+about: Ask a question
+title: ''
+labels: 'question'
+assignees: ''
+
+---
+
+Please ask questions on [Stack Overflow](https://stackoverflow.com/questions/tagged/allennlp) rather than on GitHub.  We monitor and triage questions on Stack Overflow with the AllenNLP label and questions there are more easily searchable for others.

README.md

@@ -29,8 +29,9 @@
 
 - [Website](https://allennlp.org/)
 - [Guide](https://guide.allennlp.org/)
-- [Forum](https://discourse.allennlp.org)
 - [Documentation](https://docs.allennlp.org/) ( [latest](https://docs.allennlp.org/latest/) | [stable](https://docs.allennlp.org/stable/) | [master](https://docs.allennlp.org/master/) )
+- [Forum](https://discourse.allennlp.org)
+- [Stack Overflow](https://stackoverflow.com/questions/tagged/allennlp)
 - [Contributing Guidelines](CONTRIBUTING.md)
 - [Officially Supported Models](https://github.com/allenai/allennlp-models)
     - [Pretrained Models](https://github.com/allenai/allennlp-models/blob/master/allennlp_models/pretrained.py)