Update issue templates (#2207)

* Update issue templates

Added three issue template types:
* bug reports
* feature requests
* discussion and general questions

The focus is on getting sufficient information in bug requests to help us reproduce the issue and communicating that we will be unable to generally support custom environments and participate in general discussions.

.github/ISSUE_TEMPLATE/bug_report.md

+---
+name: Bug report
+about: Report a bug with ML-Agents
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+**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
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**Environment (please complete the following information):**
+- OS + version: [e.g. Windows 10]
+- _ML-Agents version_: (e.g. ML-Agents v0.8, or latest `develop` branch from source)
+- _Environment_: (which example environment you used to reproduce the error)
+
+**NOTE:** We are unable to help reproduce bugs with custom environments.  Please attempt to reproduce your issue with one of the example environments, or provide a minimal patch to one of the environments needed to reproduce the issue.

.github/ISSUE_TEMPLATE/discussion---general-questions.md

+---
+name: Discussion / General Questions
+about: Discussion about ML-Agents, RL algorithms, or game integrations
+title: ''
+labels: discussion
+assignees: ''
+
+---
+
+Describe what you'd like to discuss.
+
+**Note**: The ML-Agents team has limited resources for education and community discussion.  We'll participate as we are able, but encourage members of the community to support one another to discuss and support one another.

.github/ISSUE_TEMPLATE/feature_request.md

+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: 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.