Add open-source boilerplate (#96)

## Summary:
There's a lot these days!  I got most of it from the GitHub docs [1], as
well as looking at other projects.  I'll probably hold off on landing
this until a couple more things land, then land this and tag v0.1.0.

[1] https://docs.github.com/en/communities

Issue: https://github.com/Khan/genqlient/issues/35

## Test plan: view in github UI


Author: benjaminjkraft

Reviewers: jvoll, StevenACoffman, aberkan, dnerdy, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: jvoll, StevenACoffman

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/96

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,23 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior. Please include whether the problem happens at code-generation time or at runtime.
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**genqlient version**
+Include your genqlient version here.
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,20 @@
+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: enhancement
+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.

.github/ISSUE_TEMPLATE/new-issue.md

@@ -1,10 +0,0 @@
----
-name: New issue
-about: gqlgen is not ready for production use!
-title: ''
-labels: ''
-assignees: ''
-
----
-
-genqlient is not ready for production use!  While bug reports and feature requests are welcome, we do not guarantee any response.

.github/ISSUE_TEMPLATE/security.md

@@ -0,0 +1,12 @@
+---
+name: Security
+about: Security issues should be reported on HackerOne.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**STOP!** Security issues should be reported to the [Khan Academy HackerOne program](https://hackerone.com/khanacademy).
+
+**DO NOT REPORT SECURITY ISSUES ON GITHUB.**