Add issue and pull request templates (#3972)

* Add issue and pull request templates

* Implement feedback

* Request new and old grammar refs

* Add note about vendor, documentation, and generated lists

* Implement @Alhadis's suggestions

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,26 @@
+<!--- Provide a general summary of the issue in the Title above -->
+
+## Preliminary Steps
+
+Please confirm you have...
+- [ ] reviewed [How Linguist Works](https://github.com/github/linguist#how-linguist-works),
+- [ ] reviewed the [Troubleshooting](https://github.com/github/linguist#troubleshooting) docs,
+- [ ] considered implementing an [override](https://github.com/github/linguist#overrides),
+- [ ] verified an issue has not already been logged for your issue ([linguist issues](https://github.com/issues?utf8=%E2%9C%93&q=is%3Aissue+repo%3Agithub/linguist)).
+
+<!-- Please review these preliminary steps before logging your issue. You may find the information referenced may answer or explain the behaviour you are seeing. It'll help us to know you've reviewed this information. -->
+
+## Problem Description
+
+<!--- Provide a more detailed introduction to the issue itself, and why you consider it to be a bug -->
+
+### URL of the affected repository:
+
+### Last modified on:
+<!-- YYYY-MM-DD -->
+
+### Expected language:
+<!-- expected language -->
+
+### Detected language:
+<!-- detected language -->