Add issue templates

This project is getting fairly popular, and I need some help sorting bug reports into:

1. Cases where `./test-image` has broken.
2. Cases where `./test-image` works, but the user is having trouble getting their own images to work.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,32 @@
+---
+name: Bug report
+about: Please let me know if something isn't working!
+title: ''
+labels: bug
+assignees: emk
+
+---
+
+Thank you for your bug report!
+
+**What did you try to do?**
+
+Please include the steps you took, including the commands you ran in your terminal.
+
+If you can simplify this or provide a link to a small public repository that demonstrates the bug, that would be extremely helpful.
+
+**What happened?**
+
+Please include the full text of any error messages.
+
+**What did you hope to happen?**
+
+A clear and concise description of what you expected to happen.
+
+**Does `./test-image` work?**
+
+If you check out the repository on a Mac or Linux system and run `./build-image`, does it succeed or fail? This will build several known-good examples from the `rust-musl-builder/examples` directory.
+
+**Additional information**
+
+Is there anything else that we should know?

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,13 @@
+---
+name: Feature request
+about: I like hearing suggestions (but I don't always have time to implement them,
+  unfortunatey).
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+**How could this project be improved?**
+
+**If you're interested in implementing this feature yourself, would you like input or mentoring?**