Update issue templates (#14)

Add template for suggesting new instructions.

Author: ngzhian

.github/ISSUE_TEMPLATE/add-new-instructions--name-.md

@@ -0,0 +1,20 @@
+---
+name: Add new instructions {name}
+about: Suggest new instructions for Relaxed SIMD
+title: ''
+labels: instruction-proposal
+assignees: ''
+
+---
+
+1. What are the instructions being proposed?
+
+2. What are the semantics of these instructions?
+
+3. How will these instructions be implemented? Give examples for at least
+   x86-64 and ARM64. Also provide reference implementation in terms of 128-bit
+   Wasm SIMD.
+
+4. How does behavior differ across processors? What new fingerprinting surfaces will be exposed?
+
+5. What use cases are there?