Add issue templates

.github/ISSUE_TEMPLATE/bug_report.md

+---
+name: Bug report
+about: Create a report to help us improve
+title: "[Bug]"
+labels: bug
+assignees: ''
+
+---
+
+**Unity bug report case number**
+Please log a bug at https://unity3d.com/unity/qa/bug-reporting and post the case number here.
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Actual behavior**
+A clear and concise description of what actually happened.
+
+**Smartphone (please complete the following information):**
+ - Device: [e.g. iPhone X]
+ - OS: [e.g. iOS 13]
+ - Unity version
+ - ARFoundation version

.github/ISSUE_TEMPLATE/feature_request.md

+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: feature request
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/ISSUE_TEMPLATE/how-to.md

+---
+name: How to
+about: How to do something in ARFoundation
+title: ''
+labels: how to
+assignees: ''
+
+---
+
+How do I...