Create ISSUE_TEMPLATE.md

Create GitHub issue template.

Template should help avoid issues being posted without enough info. Based on best practices at https://github.com/stevemao/github-issue-templates

Closes #5239

ISSUE_TEMPLATE.md

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+<!--- ** Issues are ONLY for new bugs & feature requests. ** -->
+<!--- ** For coding problems see: forum.processing.org/ ** -->
+<!--- ** Before posting, search Issues for duplicates!  ** --> 
+
+## Description
+<!--- Use a descriptive Issue Title. -->
+<!--- Give a description of the proposed change. -->
+
+## Expected Behavior
+<!--- Bug? Tell us what should happen. -->
+<!--- Improvement? Tell us how it should work. -->
+
+## Current Behavior
+<!--- Bug? What happens that is unexpected? -->
+<!--- Improvement? Explain the difference from current behavior. -->
+
+## Steps to Reproduce
+<!--- Provide an unambiguous set of steps to reproduce. -->
+<!--- Include code if relevant. -->
+1.
+2.
+3.
+
+## Context
+<!--- How has this issue affected you? What are you trying to accomplish? -->
+
+## Your Environment
+<!--- Include as many relevant details about your environment -->
+* Processing version:
+* Operating System and OS version:
+* Other information:
+
+## Possible Causes / Solutions
+<!--- Optionally, ideas on how to implement the change. -->