Add issue templates (#3248)

Add issue templates of blank issue, improvement and reporting bug in WAMR.
And fix several invalid links in ATTRIBUTIONS.md.

.github/ISSUE_TEMPLATE/blank_issue.md

@@ -0,0 +1,5 @@
+---
+name: Blank Issue
+about: Create a blank issue.
+title: ''
+---

.github/ISSUE_TEMPLATE/improvement.md

@@ -0,0 +1,28 @@
+---
+name: Improvement
+about: A feature request or code improvement.
+title: ''
+labels: ''
+assignees: ''
+---
+
+Thanks for filing a feature request! Please fill out the TODOs below.
+
+#### Feature
+
+TODO: Brief description of the feature/improvement you'd like to see in WAMR
+
+#### Benefit
+
+TODO: What is the value of adding this in WAMR? What problems does it solve?
+
+#### Implementation
+
+TODO: Do you have an implementation plan, and/or ideas for data structures or
+algorithms to use?
+
+#### Alternatives
+
+TODO: What are the alternative implementation approaches or alternative ways to
+solve the problem that this feature would solve? How do these alternatives
+compare to this proposal?

.github/ISSUE_TEMPLATE/report_bug.md

@@ -0,0 +1,36 @@
+---
+name: WAMR bug or defect report
+about: Report a bug or defect in WAMR
+title: ''
+---
+
+Thanks for filing a bug or defect report! Please fill out the TODOs below.
+
+### Subject of the issue
+
+Describe the bug or defect here.
+
+### Test case
+
+Upload the related wasm file, wast file or the source files if you can.
+
+### Your environment
+
+* Host OS
+* WAMR version, platform, cpu architecture, running mode, etc.
+
+### Steps to reproduce
+
+Tell us how to reproduce this bug or defect.
+
+### Expected behavior
+
+Tell us what should happen
+
+### Actual behavior
+
+Tell us what happens instead
+
+### Extra Info
+
+Anything else you'd like to add?