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/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.**