docs(contrib): add ISSUES doc

docs/ISSUES.md

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+# Issues
+
+The [issue tracker][issues] for hyper is where we track all features, bugs, and discuss proposals.
+
+## Labels
+
+Issues are organized with a set of labels. Most labels follow a system of being prefixed by a "type".
+
+### Area
+
+The area labels describe what part of hyper is relevant.
+
+- **A-body**: streaming request and response bodies.
+- **A-client**: the HTTP client.
+- **A-dependencies**: library dependencies.
+- **A-docs**: documentation.
+- **A-error**: error reporting and types.
+- **A-ffi**: the C API.
+- **A-http1**: the HTTP/1 specifics.
+- **A-http2**: the HTTP/2 specifics.
+- **A-server**: the HTTP server.
+- **A-tests**: the unit and integration tests.
+
+### Blocked
+
+These labels indicate an issue is "blocked" for some reason.
+
+- **B-breaking-change**: a breaking change that is waiting for the next semver-compatible release.
+- **B-rfc**: request for comments. More discussion is needed to explore the design.
+- **B-upstream**: waiting on something in a dependency or the compiler.
+
+### Effort
+
+The effort labels are a best-guess at roughly how much effort and knowledge of hyper is needed to accomplish the task.
+
+- **E-easy**: a great starting point for a new contributor.
+- **E-medium**: some knowledge of how hyper internals work would be useful.
+- **E-hard**: likely requires a deeper understanding of how hyper internals work.
+
+### Severity
+
+The severity marks how _severe_ the issue is. Note this isn't "importance" or "priority".
+
+- **S-bug**: something is wrong, this is bad!
+- **S-feature**: this is a new feature request, adding something new.
+- **S-performance**: make existing working code go faster.
+- **S-refactor**: improve internal code to help readability and maintenance.
+
+[issues]: https://github.com/hyperium/hyper/issues