Update issue templates

Author: GiggleLiu

.github/ISSUE_TEMPLATE/problem.md

@@ -0,0 +1,26 @@
+---
+name: Problem
+about: For contributors to propose a new model problem
+title: "[Model] The problem you want to introduce to this repo"
+labels: ''
+assignees: ''
+
+---
+
+## Description
+[Problem name, e.g. Maximum Independent Set]
+
+[Description, with clear annotation of information source. e.g. 
+Given a Graph G=(V,E), an independent set of vertices, i.e. a subset of V such that no two vertices in it are joined by an edge in E. To goal is to find the the independent set with maximum cardinality.
+
+Ref: https://www.csc.kth.se/~viggo/wwwcompendium/node34.html
+]
+
+## Schema
+[JSON Schema description, e.g.
+Type name: MaximumIndependentSet
+Vartiants: Different graph topolofy, weighted or unweighted (implemented as type parameters)
+Fields:
+- `graph`, a graph description
+- `weights`, a vector of numbers, for weights on vertices
+]